Package analysis

Overview ▾

Package analysis defines the interface between a modular static analysis and an analysis driver program.

Background

A static analysis is a function that inspects a package of Go code and reports a set of diagnostics (typically mistakes in the code), and perhaps produces other results as well, such as suggested refactorings or other facts. An analysis that reports mistakes is informally called a "checker". For example, the printf checker reports mistakes in fmt.Printf format strings.

A "modular" analysis is one that inspects one package at a time but can save information from a lower-level package and use it when inspecting a higher-level package, analogous to separate compilation in a toolchain. The printf checker is modular: when it discovers that a function such as log.Fatalf delegates to fmt.Printf, it records this fact, and checks calls to that function too, including calls made from another package.

By implementing a common interface, checkers from a variety of sources can be easily selected, incorporated, and reused in a wide range of driver programs including command-line tools (such as vet), text editors and IDEs, build and test systems (such as go build, Bazel, or Buck), test frameworks, code review tools, code-base indexers (such as SourceGraph), documentation viewers (such as godoc), batch pipelines for large code bases, and so on.

Analyzer

The primary type in the API is Analyzer. An Analyzer statically describes an analysis function: its name, documentation, flags, relationship to other analyzers, and of course, its logic.

To define an analysis, a user declares a (logically constant) variable of type Analyzer. Here is a typical example from one of the analyzers in the go/analysis/passes/ subdirectory:

package unusedresult

var Analyzer = &analysis.Analyzer{
	Name: "unusedresult",
	Doc:  "check for unused results of calls to some functions",
	Run:  run,
	...
}

func run(pass *analysis.Pass) (interface{}, error) {
	...
}

An analysis driver is a program such as vet that runs a set of analyses and prints the diagnostics that they report. The driver program must import the list of Analyzers it needs. Typically each Analyzer resides in a separate package. To add a new Analyzer to an existing driver, add another item to the list:

import ( "unusedresult"; "nilness"; "printf" )

var analyses = []*analysis.Analyzer{
	unusedresult.Analyzer,
	nilness.Analyzer,
	printf.Analyzer,
}

A driver may use the name, flags, and documentation to provide on-line help that describes the analyses it performs. The doc comment contains a brief one-line summary, optionally followed by paragraphs of explanation.

The Analyzer type has more fields besides those shown above:

type Analyzer struct {
	Name             string
	Doc              string
	Flags            flag.FlagSet
	Run              func(*Pass) (interface{}, error)
	RunDespiteErrors bool
	ResultType       reflect.Type
	Requires         []*Analyzer
	FactTypes        []Fact
}

The Flags field declares a set of named (global) flag variables that control analysis behavior. Unlike vet, analysis flags are not declared directly in the command line FlagSet; it is up to the driver to set the flag variables. A driver for a single analysis, a, might expose its flag f directly on the command line as -f, whereas a driver for multiple analyses might prefix the flag name by the analysis name (-a.f) to avoid ambiguity. An IDE might expose the flags through a graphical interface, and a batch pipeline might configure them from a config file. See the "findcall" analyzer for an example of flags in action.

The RunDespiteErrors flag indicates whether the analysis is equipped to handle ill-typed code. If not, the driver will skip the analysis if there were parse or type errors. The optional ResultType field specifies the type of the result value computed by this analysis and made available to other analyses. The Requires field specifies a list of analyses upon which this one depends and whose results it may access, and it constrains the order in which a driver may run analyses. The FactTypes field is discussed in the section on Modularity. The analysis package provides a Validate function to perform basic sanity checks on an Analyzer, such as that its Requires graph is acyclic, its fact and result types are unique, and so on.

Finally, the Run field contains a function to be called by the driver to execute the analysis on a single package. The driver passes it an instance of the Pass type.

Pass

A Pass describes a single unit of work: the application of a particular Analyzer to a particular package of Go code. The Pass provides information to the Analyzer's Run function about the package being analyzed, and provides operations to the Run function for reporting diagnostics and other information back to the driver.

type Pass struct {
	Fset         *token.FileSet
	Files        []*ast.File
	OtherFiles   []string
	IgnoredFiles []string
	Pkg          *types.Package
	TypesInfo    *types.Info
	ResultOf     map[*Analyzer]interface{}
	Report       func(Diagnostic)
	...
}

The Fset, Files, Pkg, and TypesInfo fields provide the syntax trees, type information, and source positions for a single package of Go code.

The OtherFiles field provides the names of non-Go files such as assembly that are part of this package. Similarly, the IgnoredFiles field provides the names of Go and non-Go source files that are not part of this package with the current build configuration but may be part of other build configurations. The contents of these files may be read using Pass.ReadFile; see the "asmdecl" or "buildtags" analyzers for examples of loading non-Go files and reporting diagnostics against them.

The ResultOf field provides the results computed by the analyzers required by this one, as expressed in its Analyzer.Requires field. The driver runs the required analyzers first and makes their results available in this map. Each Analyzer must return a value of the type described in its Analyzer.ResultType field. For example, the "ctrlflow" analyzer returns a *ctrlflow.CFGs, which provides a control-flow graph for each function in the package (see golang.org/x/tools/go/cfg); the "inspect" analyzer returns a value that enables other Analyzers to traverse the syntax trees of the package more efficiently; and the "buildssa" analyzer constructs an SSA-form intermediate representation. Each of these Analyzers extends the capabilities of later Analyzers without adding a dependency to the core API, so an analysis tool pays only for the extensions it needs.

The Report function emits a diagnostic, a message associated with a source position. For most analyses, diagnostics are their primary result. For convenience, Pass provides a helper method, Reportf, to report a new diagnostic by formatting a string. Diagnostic is defined as:

type Diagnostic struct {
	Pos      token.Pos
	Category string // optional
	Message  string
}

The optional Category field is a short identifier that classifies the kind of message when an analysis produces several kinds of diagnostic.

The Diagnostic struct does not have a field to indicate its severity because opinions about the relative importance of Analyzers and their diagnostics vary widely among users. The design of this framework does not hold each Analyzer responsible for identifying the severity of its diagnostics. Instead, we expect that drivers will allow the user to customize the filtering and prioritization of diagnostics based on the producing Analyzer and optional Category, according to the user's preferences.

Most Analyzers inspect typed Go syntax trees, but a few, such as asmdecl and buildtag, inspect the raw text of Go source files or even non-Go files such as assembly. To report a diagnostic against a line of a raw text file, use the following sequence:

content, err := pass.ReadFile(filename)
if err != nil { ... }
tf := fset.AddFile(filename, -1, len(content))
tf.SetLinesForContent(content)
...
pass.Reportf(tf.LineStart(line), "oops")

Modular analysis with Facts

To improve efficiency and scalability, large programs are routinely built using separate compilation: units of the program are compiled separately, and recompiled only when one of their dependencies changes; independent modules may be compiled in parallel. The same technique may be applied to static analyses, for the same benefits. Such analyses are described as "modular".

A compiler’s type checker is an example of a modular static analysis. Many other checkers we would like to apply to Go programs can be understood as alternative or non-standard type systems. For example, vet's printf checker infers whether a function has the "printf wrapper" type, and it applies stricter checks to calls of such functions. In addition, it records which functions are printf wrappers for use by later analysis passes to identify other printf wrappers by induction. A result such as “f is a printf wrapper” that is not interesting by itself but serves as a stepping stone to an interesting result (such as a diagnostic) is called a Fact.

The analysis API allows an analysis to define new types of facts, to associate facts of these types with objects (named entities) declared within the current package, or with the package as a whole, and to query for an existing fact of a given type associated with an object or package.

An Analyzer that uses facts must declare their types:

var Analyzer = &analysis.Analyzer{
	Name:      "printf",
	FactTypes: []analysis.Fact{new(isWrapper)},
	...
}

type isWrapper struct{} // => *types.Func f “is a printf wrapper”

The driver program ensures that facts for a pass’s dependencies are generated before analyzing the package and is responsible for propagating facts from one package to another, possibly across address spaces. Consequently, Facts must be serializable. The API requires that drivers use the gob encoding, an efficient, robust, self-describing binary protocol. A fact type may implement the GobEncoder/GobDecoder interfaces if the default encoding is unsuitable. Facts should be stateless. Because serialized facts may appear within build outputs, the gob encoding of a fact must be deterministic, to avoid spurious cache misses in build systems that use content-addressable caches. The driver makes a single call to the gob encoder for all facts exported by a given analysis pass, so that the topology of shared data structures referenced by multiple facts is preserved.

The Pass type has functions to import and export facts, associated either with an object or with a package:

type Pass struct {
	...
	ExportObjectFact func(types.Object, Fact)
	ImportObjectFact func(types.Object, Fact) bool

	ExportPackageFact func(fact Fact)
	ImportPackageFact func(*types.Package, Fact) bool
}

An Analyzer may only export facts associated with the current package or its objects, though it may import facts from any package or object that is an import dependency of the current package.

Conceptually, ExportObjectFact(obj, fact) inserts fact into a hidden map keyed by the pair (obj, TypeOf(fact)), and the ImportObjectFact function retrieves the entry from this map and copies its value into the variable pointed to by fact. This scheme assumes that the concrete type of fact is a pointer; this assumption is checked by the Validate function. See the "printf" analyzer for an example of object facts in action.

Some driver implementations (such as those based on Bazel and Blaze) do not currently apply analyzers to packages of the standard library. Therefore, for best results, analyzer authors should not rely on analysis facts being available for standard packages. For example, although the printf checker is capable of deducing during analysis of the log package that log.Printf is a printf wrapper, this fact is built in to the analyzer so that it correctly checks calls to log.Printf even when run in a driver that does not apply it to standard packages. We would like to remove this limitation in future.

Testing an Analyzer

The analysistest subpackage provides utilities for testing an Analyzer. In a few lines of code, it is possible to run an analyzer on a package of testdata files and check that it reported all the expected diagnostics and facts (and no more). Expectations are expressed using "// want ..." comments in the input code.

Standalone commands

Analyzers are provided in the form of packages that a driver program is expected to import. The vet command imports a set of several analyzers, but users may wish to define their own analysis commands that perform additional checks. To simplify the task of creating an analysis command, either for a single analyzer or for a whole suite, we provide the singlechecker and multichecker subpackages.

The singlechecker package provides the main function for a command that runs one analyzer. By convention, each analyzer such as go/analysis/passes/findcall should be accompanied by a singlechecker-based command such as go/analysis/passes/findcall/cmd/findcall, defined in its entirety as:

package main

import (
	"golang.org/x/tools/go/analysis/passes/findcall"
	"golang.org/x/tools/go/analysis/singlechecker"
)

func main() { singlechecker.Main(findcall.Analyzer) }

A tool that provides multiple analyzers can use multichecker in a similar way, giving it the list of Analyzers.

Index ▾

Package files

analysis.go diagnostic.go doc.go validate.go