Source code and community space of 📖 100 Go Mistakes and How to Avoid Them, published by Manning in 2022.

100 Go Mistakes and How to Avoid Them shows you how to replace common programming problems in Go with idiomatic, expressive code. In it, you’ll explore dozens of interesting examples and case studies as you learn to spot mistakes that might appear in your own applications.

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This section contains a summary of the 100 mistakes in the book. Meanwhile, it's also a section open to the community. If you believe that a mistake should be added, please create a community mistake issue.

• Code and Project Organization
  • Unintended variable shadowing
  • Unnecessary nested code
  • Misusing init functions
  • Overusing getters and setters
  • Interface pollution
  • Interface on the producer side
  • Returning interfaces
  • any says nothing
  • Being confused about when to use generics (Read the full excerpt 👀)
  • Not being aware of the possible problems with type embedding
  • Not using the functional options pattern
  • Project misorganization
  • Creating utility packages
  • Ignoring package name collisions
  • Missing code documentation
  • Not using linters
• Data Types
  • Creating confusion with octal literals
  • Neglecting integer overflows
  • Not understanding floating-points
  • Not understanding slice length and capacity (Read the full excerpt 👀)
  • Inefficient slice initialization
  • Being confused about nil vs. empty slice
  • Not properly checking if a slice is empty
  • Not making slice copies correctly
  • Unexpected side effects using slice append
  • Slice and memory leaks
  • Inefficient map initialization
  • Map and memory leaks (Read the full excerpt 👀)
  • Comparing values incorrectly
• Control Structures
  • Ignoring that elements are copied in range loops
  • Ignoring how arguments are evaluated in range loops
  • Ignoring the impacts of using pointer elements in range loops
  • Making wrong assumptions during map iterations
  • Ignoring how the break statement works
  • Using defer inside a loop
• Strings
  • Not understanding the concept of rune
  • Inaccurate string iteration
  • Misusing trim functions
  • Under-optimized strings concatenation
  • Useless string conversions
  • Substring and memory leaks
• Functions and Methods
  • Not knowing which type of receiver to use
  • Never using named result parameters
  • Unintended side effects with named result parameters
  • Returning a nil receiver
  • Using a filename as a function input
  • Ignoring how defer arguments and receivers are evaluated
• Error Management
  • Panicking
  • Ignoring when to wrap an error
  • Comparing an error type inaccurately
  • Comparing an error value inaccurately
  • Handling an error twice
  • Not handling an error
  • Not handling defer errors
• Concurrency: Foundations
  • Mixing up concurrency and parallelism
  • Thinking concurrency is always faster (Read the full excerpt 👀)
  • Being puzzled about when to use channels or mutexes
  • Not understanding race problems
  • Not understanding the concurrency impacts of a workload type
  • Misunderstanding Go contexts
• Concurrency: Practice
  • Propagating an inappropriate context
  • Starting a goroutine without knowing when to stop it
  • Not being careful with goroutines and loop variables
  • Expecting a deterministic behavior using select and channels
  • Not using notification channels
  • Not using nil channels
  • Being puzzled about channel size
  • Forgetting about possible side effects with string formatting
  • Creating data races with append
  • Using mutexes inaccurately with slices and maps
  • Misusing sync.WaitGroup
  • Forgetting about sync.Cond
  • Not using errgroup
  • Copying a sync type
• Standard Library
  • Providing a wrong time duration
  • time.After and memory leaks
  • JSON handling common mistakes
  • Common SQL mistakes
  • Not closing transient resources
  • Forgetting the return statement after replying to an HTTP request
  • Using the default HTTP client and server
• Testing
  • Not categorizing tests
  • Not enabling the race flag
  • Not using test execution modes
  • Not using table-driven tests
  • Sleeping in unit tests
  • Not dealing with the time API efficiently
  • Not using testing utility packages
  • Writing inaccurate benchmarks (Read the full excerpt 👀)
  • Not exploring all the Go testing features
• Optimizations
  • Not understanding CPU caches
  • Writing concurrent code that leads to false sharing
  • Not taking into account instruction-level parallelism
  • Not being aware of data alignment
  • Not understanding stack vs. heap
  • Not knowing how to reduce allocations
  • Not relying on inlining
  • Not using Go diagnostics tooling (Read the full excerpt 👀)
  • Not understanding how the GC works
  • Not understanding the impacts of running Go in Docker and Kubernetes

Avoiding shadowed variables can help prevent mistakes like referencing the wrong variable or confusing readers.

Avoiding nested levels and keeping the happy path aligned on the left makes building a mental code model easier.

When initializing variables, remember that init functions have limited error handling and make state handling and testing more complex. In most cases, initializations should be handled as specific functions.

Forcing the use of getters and setters isn’t idiomatic in Go. Being pragmatic and finding the right balance between efficiency and blindly following certain idioms should be the way to go.

Abstractions should be discovered, not created. To prevent unnecessary complexity, create an interface when you need it and not when you foresee needing it, or if you can at least prove the abstraction to be a valid one.

Keeping interfaces on the client side avoids unnecessary abstractions.

To prevent being restricted in terms of flexibility, a function shouldn’t return interfaces but concrete implementations in most cases. Conversely, a function should accept interfaces whenever possible.

Only use any if you need to accept or return any possible type, such as json.Marshal. Otherwise, any doesn’t provide meaningful information and can lead to compile-time issues by allowing a caller to call methods with any data type.

Relying on generics and type parameters can prevent writing boilerplate code to factor out elements or behaviors. However, do not use type parameters prematurely, but only when you see a concrete need for them. Otherwise, they introduce unnecessary abstractions and complexity.

Using type embedding can also help avoid boilerplate code; however, ensure that doing so doesn’t lead to visibility issues where some fields should have remained hidden.

To handle options conveniently and in an API-friendly manner, use the functional options pattern.

Following a layout such as project-layout can be a good way to start structuring Go projects, especially if you are looking for existing conventions to standardize a new project.

Naming is a critical piece of application design. Creating packages such as common, util, and shared doesn’t bring much value for the reader. Refactor such packages into meaningful and specific package names.

To avoid naming collisions between variables and packages, leading to confusion or perhaps even bugs, use unique names for each one. If this isn’t feasible, use an import alias to change the qualifier to differentiate the package name from the variable name, or think of a better name.

To help clients and maintainers understand your code’s purpose, document exported elements.

To improve code quality and consistency, use linters and formatters.

When reading existing code, bear in mind that integer literals starting with 0 are octal numbers. Also, to improve readability, make octal integers explicit by prefixing them with 0o.

Because integer overflows and underflows are handled silently in Go, you can implement your own functions to catch them.

Making floating-point comparisons within a given delta can ensure that your code is portable.

When performing addition or subtraction, group the operations with a similar order of magnitude to favor accuracy. Also, perform multiplication and division before addition and subtraction.

Understanding the difference between slice length and capacity should be part of a Go developer’s core knowledge. The slice length is the number of available elements in the slice, whereas the slice capacity is the number of elements in the backing array.

When creating a slice, initialize it with a given length or capacity if its length is already known. This reduces the number of allocations and improves performance. The same logic goes for maps, and you need to initialize their size.

To prevent common confusions such as when using the encoding/json or the reflect package, you need to understand the difference between nil and empty slices. Both are zero-length, zero-capacity slices, but only a nil slice doesn’t require allocation.

To check if a slice doesn’t contain any element, check its length. This check works regardless of whether the slice is nil or empty. The same goes for maps.

To design unambiguous APIs, you shouldn’t distinguish between nil and empty slices.

To copy one slice to another using the copy built-in function, remember that the number of copied elements corresponds to the minimum between the two slice’s lengths.

Using copy or the full slice expression is a way to prevent append from creating conflicts if two different functions use slices backed by the same array. However, only a slice copy prevents memory leaks if you want to shrink a large slice.

Working with a slice of pointers or structs with pointer fields, you can avoid memory leaks by marking as nil the elements excluded by a slicing operation.

See #21.

A map can always grow in memory, but it never shrinks. Hence, if it leads to some memory issues, you can try different options, such as forcing Go to recreate the map or using pointers.

To compare types in Go, you can use the == and != operators if two types are comparable: Booleans, numerals, strings, pointers, channels, and structs are composed entirely of comparable types. Otherwise, you can either use reflect.DeepEqual and pay the price of reflection or use custom implementations and libraries.

The value element in a range loop is a copy. Therefore, to mutate a struct, for example, access it via its index or via a classic for loop (unless the element or the field you want to modify is a pointer).

Understanding that the expression passed to the range operator is evaluated only once before the beginning of the loop can help you avoid common mistakes such as inefficient assignment in channel or slice iteration.

Using a local variable or accessing an element using an index, you can prevent mistakes while copying pointers inside a loop.

To ensure predictable outputs when using maps, remember that a map data structure:

• Doesn’t order the data by keys
• Doesn’t preserve the insertion order
• Doesn’t have a deterministic iteration order
• Doesn’t guarantee that an element added during an iteration will be produced during this iteration

Using break or continue with a label enforces breaking a specific statement. This can be helpful with switch or select statements inside loops.

Extracting loop logic inside a function leads to executing a defer statement at the end of each iteration.

Understanding that a rune corresponds to the concept of a Unicode code point and that it can be composed of multiple bytes should be part of the Go developer’s core knowledge to work accurately with strings.

Iterating on a string with the range operator iterates on the runes with the index corresponding to the starting index of the rune’s byte sequence. To access a specific rune index (such as the third rune), convert the string into a []rune.

strings.TrimRight/strings.TrimLeft removes all the trailing/leading runes contained in a given set, whereas strings.TrimSuffix/strings.TrimPrefix returns a string without a provided suffix/prefix.

Concatenating a list of strings should be done with strings.Builder to prevent allocating a new string during each iteration.

Remembering that the bytes package offers the same operations as the strings package can help avoid extra byte/string conversions.

Using copies instead of substrings can prevent memory leaks, as the string returned by a substring operation will be backed by the same byte array.

The decision whether to use a value or a pointer receiver should be made based on factors such as the type, whether it has to be mutated, whether it contains a field that can’t be copied, and how large the object is. When in doubt, use a pointer receiver.

Using named result parameters can be an efficient way to improve the readability of a function/method, especially if multiple result parameters have the same type. In some cases, this approach can also be convenient because named result parameters are initialized to their zero value. But be cautious about potential side effects.

See #43.

When returning an interface, be cautious about returning not a nil pointer but an explicit nil value. Otherwise, unintended consequences may result because the caller will receive a non-nil value.

Designing functions to receive io.Reader types instead of filenames improves the reusability of a function and makes testing easier.

Passing a pointer to a defer function and wrapping a call inside a closure are two possible solutions to overcome the immediate evaluation of arguments and receivers.

Using panic is an option to deal with errors in Go. However, it should only be used sparingly in unrecoverable conditions: for example, to signal a programmer error or when you fail to load a mandatory dependency.

Wrapping an error allows you to mark an error and/or provide additional context. However, error wrapping creates potential coupling as it makes the source error available for the caller. If you want to prevent that, don’t use error wrapping.

If you use Go 1.13 error wrapping with the %w directive and fmt.Errorf, comparing an error against a type or a value has to be done using errors.As or errors.Is, respectively. Otherwise, if the returned error you want to check is wrapped, it will fail the checks.

See #50.

To convey an expected error, use error sentinels (error values). An unexpected error should be a specific error type.

In most situations, an error should be handled only once. Logging an error is handling an error. Therefore, you have to choose between logging or returning an error. In many cases, error wrapping is the solution as it allows you to provide additional context to an error and return the source error.

Ignoring an error, whether during a function call or in a defer function, should be done explicitly using the blank identifier. Otherwise, future readers may be confused about whether it was intentional or a miss.

In many cases, you shouldn’t ignore an error returned by a defer function. Either handle it directly or propagate it to the caller, depending on the context. If you want to ignore it, use the blank identifier.

Understanding the fundamental differences between concurrency and parallelism is a cornerstone of the Go developer’s knowledge. Concurrency is about structure, whereas parallelism is about execution.

To be a proficient developer, you must acknowledge that concurrency isn’t always faster. Solutions involving parallelization of minimal workloads may not necessarily be faster than a sequential implementation. Benchmarking sequential versus concurrent solutions should be the way to validate assumptions.

Being aware of goroutine interactions can also be helpful when deciding between channels and mutexes. In general, parallel goroutines require synchronization and hence mutexes. Conversely, concurrent goroutines generally require coordination and orchestration and hence channels.

Being proficient in concurrency also means understanding that data races and race conditions are different concepts. Data races occur when multiple goroutines simultaneously access the same memory location and at least one of them is writing. Meanwhile, being data-race-free doesn’t necessarily mean deterministic execution. When a behavior depends on the sequence or the timing of events that can’t be controlled, this is a race condition.

Understanding the Go memory model and the underlying guarantees in terms of ordering and synchronization is essential to prevent possible data races and/or race conditions.

When creating a certain number of goroutines, consider the workload type. Creating CPU-bound goroutines means bounding this number close to the GOMAXPROCS variable (based by default on the number of CPU cores on the host). Creating I/O-bound goroutines depends on other factors, such as the external system.

Go contexts are also one of the cornerstones of concurrency in Go. A context allows you to carry a deadline, a cancellation signal, and/or a list of keys-values.

Understanding the conditions when a context can be canceled should matter when propagating it: for example, an HTTP handler canceling the context when the response has been sent.

Avoiding leaks means being mindful that whenever a goroutine is started, you should have a plan to stop it eventually.

To avoid bugs with goroutines and loop variables, create local variables or call functions instead of closures.

Understanding that select with multiple channels chooses the case randomly if multiple options are possible prevents making wrong assumptions that can lead to subtle concurrency bugs.

Send notifications using a chan struct{} type.

Using nil channels should be part of your concurrency toolset because it allows you to remove cases from select statements, for example.

Carefully decide on the right channel type to use, given a problem. Only unbuffered channels provide strong synchronization guarantees.

You should have a good reason to specify a channel size other than one for buffered channels.

Being aware that string formatting may lead to calling existing functions means watching out for possible deadlocks and other data races.

Calling append isn’t always data-race-free; hence, it shouldn’t be used concurrently on a shared slice.

Remembering that slices and maps are pointers can prevent common data races.

To accurately use sync.WaitGroup, call the Add method before spinning up goroutines.

You can send repeated notifications to multiple goroutines with sync.Cond.

You can synchronize a group of goroutines and handle errors and contexts with the errgroup package.

sync types shouldn’t be copied.

Remain cautious with functions accepting a time.Duration. Even though passing an integer is allowed, strive to use the time API to prevent any possible confusion.

Avoiding calls to time.After in repeated functions (such as loops or HTTP handlers) can avoid peak memory consumption. The resources created by time.After are released only when the timer expires.

• Unexpected behavior because of type embedding

  Be careful about using embedded fields in Go structs. Doing so may lead to sneaky bugs like an embedded time.Time field implementing the json.Marshaler interface, hence overriding the default marshaling behavior.

• JSON and the monotonic clock

  When comparing two time.Time structs, recall that time.Time contains both a wall clock and a monotonic clock, and the comparison using the == operator is done on both clocks.

• Map of any

  To avoid wrong assumptions when you provide a map while unmarshaling JSON data, remember that numerics are converted to float64 by default.

• Forgetting that sql.Open doesn't necessarily establish connections to a database

  Call the Ping or PingContext method if you need to test your configuration and make sure a database is reachable.

• Forgetting about connections pooling

  Configure the database connection parameters for production-grade applications.

• Not using prepared statements

  Using SQL prepared statements makes queries more efficient and more secure.

• Mishandling null values

  Deal with nullable columns in tables using pointers or sql.NullXXX types.

• Not handling rows iteration errors

  Call the Err method of sql.Rows after row iterations to ensure that you haven’t missed an error while preparing the next row.

Eventually close all structs implementing io.Closer to avoid possible leaks.

To avoid unexpected behaviors in HTTP handler implementations, make sure you don’t miss the return statement if you want a handler to stop after http.Error.

For production-grade applications, don’t use the default HTTP client and server implementations. These implementations are missing timeouts and behaviors that should be mandatory in production.

Categorizing tests using build flags, environment variables, or short mode makes the testing process more efficient. You can create test categories using build flags or environment variables (for example, unit versus integration tests) and differentiate short from long-running tests to decide which kinds of tests to execute.

Enabling the -race flag is highly recommended when writing concurrent applications. Doing so allows you to catch potential data races that can lead to software bugs.

Using the -parallel flag is an efficient way to speed up tests, especially long-running ones.

Use the -shuffle flag to help ensure that a test suite doesn’t rely on wrong assumptions that could hide bugs.

Table-driven tests are an efficient way to group a set of similar tests to prevent code duplication and make future updates easier to handle.

Avoid sleeps using synchronization to make a test less flaky and more robust. If synchronization isn’t possible, consider a retry approach.

Understanding how to deal with functions using the time API is another way to make a test less flaky. You can use standard techniques such as handling the time as part of a hidden dependency or asking clients to provide it.

The httptest package is helpful for dealing with HTTP applications. It provides a set of utilities to test both clients and servers.

The iotest package helps write io.Reader and test that an application is tolerant to errors.

• Not resetting or pausing the timer

  Use time methods to preserve the accuracy of a benchmark.

• Making wrong assumptions about micro-benchmarks

  Increasing benchtime or using tools such as benchstat can be helpful when dealing with micro-benchmarks.

  Be careful with the results of a micro-benchmark if the system that ends up running the application is different from the one running the micro-benchmark.

• Not being careful about compiler optimizations

  Make sure the function under test leads to a side effect, to prevent compiler optimizations from fooling you about the benchmark results.

• Being fooled by the observer effect

  To prevent the observer effect, force a benchmark to re-create the data used by a CPU-bound function.

• Code coverage

  Use code coverage with the -coverprofile flag to quickly see which part of the code needs more attention.

• Testing from a different package

  Place unit tests in a different package to enforce writing tests that focus on an exposed behavior, not internals.

• Utility functions

  Handling errors using the *testing.T variable instead of the classic if err != nil makes code shorter and easier to read.

• Setup and teardown

  You can use setup and teardown functions to configure a complex environment, such as in the case of integration tests.

• CPU architecture

  Understanding how to use CPU caches is important for optimizing CPU-bound applications because the L1 cache is about 50 to 100 times faster than the main memory.

• Cache line

  Being conscious of the cache line concept is critical to understanding how to organize data in data-intensive applications. A CPU doesn’t fetch memory word by word; instead, it usually copies a memory block to a 64-byte cache line. To get the most out of each individual cache line, enforce spatial locality.

• Slice of structs vs. struct of slices

• Predictability

  Making code predictable for the CPU can also be an efficient way to optimize certain functions. For example, a unit or constant stride is predictable for the CPU, but a non-unit stride (for example, a linked list) isn’t predictable.

• Cache placement policy

  To avoid a critical stride, hence utilizing only a tiny portion of the cache, be aware that caches are partitioned.

Knowing that lower levels of CPU caches aren’t shared across all the cores helps avoid performance-degrading patterns such as false sharing while writing concurrency code. Sharing memory is an illusion.

Use instruction-level parallelism (ILP) to optimize specific parts of your code to allow a CPU to execute as many parallel instructions as possible. Identifying data hazards is one of the main steps.

You can avoid common mistakes by remembering that in Go, basic types are aligned with their own size. For example, keep in mind that reorganizing the fields of a struct by size in descending order can lead to more compact structs (less memory allocation and potentially a better spatial locality).

Understanding the fundamental differences between heap and stack should also be part of your core knowledge when optimizing a Go application. Stack allocations are almost free, whereas heap allocations are slower and rely on the GC to clean the memory.

Reducing allocations is also an essential aspect of optimizing a Go application. This can be done in different ways, such as designing the API carefully to prevent sharing up, understanding the common Go compiler optimizations, and using sync.Pool.

Use the fast-path inlining technique to efficiently reduce the amortized time to call a function.

Rely on profiling and the execution tracer to understand how an application performs and the parts to optimize.

Understanding how to tune the GC can lead to multiple benefits such as handling sudden load increases more efficiently.

To help avoid CPU throttling when deployed in Docker and Kubernetes, keep in mind that Go isn’t CFS-aware.

Teiva Harsanyi is a senior software engineer at Docker 🐳. He worked in various domains, including insurance, transportation, and safety-critical industries like air traffic management. He is passionate about Go and how to design and implement reliable applications.

This should be the required reading for all Golang developers before they touch code in Production... It's the Golang equivalent of the legendary 'Effective Java' by Joshua Bloch.

– Neeraj Shah

This unique book teaches you good habits by helping you identify bad ones. Harsanyi's writing style is engaging, the examples relevant, and his insights useful. I thought it was a great read, and I think you will too.

– Thad Meyer

Not having this will be the 101st mistake a Go programmer could make.

– Anupam Sengupta

• How to make mistakes in Go (Go Time - episode #190)

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• 8LU - 100% Test Coverage: YouTube