| // Copyright 2015 The Go Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style |
| // license that can be found in the LICENSE file. |
| |
| // Package loader loads a complete Go program from source code, parsing |
| // and type-checking the initial packages plus their transitive closure |
| // of dependencies. The ASTs and the derived facts are retained for |
| // later use. |
| // |
| // THIS INTERFACE IS EXPERIMENTAL AND IS LIKELY TO CHANGE. |
| // |
| // The package defines two primary types: Config, which specifies a |
| // set of initial packages to load and various other options; and |
| // Program, which is the result of successfully loading the packages |
| // specified by a configuration. |
| // |
| // The configuration can be set directly, but *Config provides various |
| // convenience methods to simplify the common cases, each of which can |
| // be called any number of times. Finally, these are followed by a |
| // call to Load() to actually load and type-check the program. |
| // |
| // var conf loader.Config |
| // |
| // // Use the command-line arguments to specify |
| // // a set of initial packages to load from source. |
| // // See FromArgsUsage for help. |
| // rest, err := conf.FromArgs(os.Args[1:], wantTests) |
| // |
| // // Parse the specified files and create an ad hoc package with path "foo". |
| // // All files must have the same 'package' declaration. |
| // conf.CreateFromFilenames("foo", "foo.go", "bar.go") |
| // |
| // // Create an ad hoc package with path "foo" from |
| // // the specified already-parsed files. |
| // // All ASTs must have the same 'package' declaration. |
| // conf.CreateFromFiles("foo", parsedFiles) |
| // |
| // // Add "runtime" to the set of packages to be loaded. |
| // conf.Import("runtime") |
| // |
| // // Adds "fmt" and "fmt_test" to the set of packages |
| // // to be loaded. "fmt" will include *_test.go files. |
| // conf.ImportWithTests("fmt") |
| // |
| // // Finally, load all the packages specified by the configuration. |
| // prog, err := conf.Load() |
| // |
| // See examples_test.go for examples of API usage. |
| // |
| // |
| // CONCEPTS AND TERMINOLOGY |
| // |
| // The WORKSPACE is the set of packages accessible to the loader. The |
| // workspace is defined by Config.Build, a *build.Context. The |
| // default context treats subdirectories of $GOROOT and $GOPATH as |
| // packages, but this behavior may be overridden. |
| // |
| // An AD HOC package is one specified as a set of source files on the |
| // command line. In the simplest case, it may consist of a single file |
| // such as $GOROOT/src/net/http/triv.go. |
| // |
| // EXTERNAL TEST packages are those comprised of a set of *_test.go |
| // files all with the same 'package foo_test' declaration, all in the |
| // same directory. (go/build.Package calls these files XTestFiles.) |
| // |
| // An IMPORTABLE package is one that can be referred to by some import |
| // spec. Every importable package is uniquely identified by its |
| // PACKAGE PATH or just PATH, a string such as "fmt", "encoding/json", |
| // or "cmd/vendor/golang.org/x/arch/x86/x86asm". A package path |
| // typically denotes a subdirectory of the workspace. |
| // |
| // An import declaration uses an IMPORT PATH to refer to a package. |
| // Most import declarations use the package path as the import path. |
| // |
| // Due to VENDORING (https://golang.org/s/go15vendor), the |
| // interpretation of an import path may depend on the directory in which |
| // it appears. To resolve an import path to a package path, go/build |
| // must search the enclosing directories for a subdirectory named |
| // "vendor". |
| // |
| // ad hoc packages and external test packages are NON-IMPORTABLE. The |
| // path of an ad hoc package is inferred from the package |
| // declarations of its files and is therefore not a unique package key. |
| // For example, Config.CreatePkgs may specify two initial ad hoc |
| // packages, both with path "main". |
| // |
| // An AUGMENTED package is an importable package P plus all the |
| // *_test.go files with same 'package foo' declaration as P. |
| // (go/build.Package calls these files TestFiles.) |
| // |
| // The INITIAL packages are those specified in the configuration. A |
| // DEPENDENCY is a package loaded to satisfy an import in an initial |
| // package or another dependency. |
| // |
| package loader |
| |
| // IMPLEMENTATION NOTES |
| // |
| // 'go test', in-package test files, and import cycles |
| // --------------------------------------------------- |
| // |
| // An external test package may depend upon members of the augmented |
| // package that are not in the unaugmented package, such as functions |
| // that expose internals. (See bufio/export_test.go for an example.) |
| // So, the loader must ensure that for each external test package |
| // it loads, it also augments the corresponding non-test package. |
| // |
| // The import graph over n unaugmented packages must be acyclic; the |
| // import graph over n-1 unaugmented packages plus one augmented |
| // package must also be acyclic. ('go test' relies on this.) But the |
| // import graph over n augmented packages may contain cycles. |
| // |
| // First, all the (unaugmented) non-test packages and their |
| // dependencies are imported in the usual way; the loader reports an |
| // error if it detects an import cycle. |
| // |
| // Then, each package P for which testing is desired is augmented by |
| // the list P' of its in-package test files, by calling |
| // (*types.Checker).Files. This arrangement ensures that P' may |
| // reference definitions within P, but P may not reference definitions |
| // within P'. Furthermore, P' may import any other package, including |
| // ones that depend upon P, without an import cycle error. |
| // |
| // Consider two packages A and B, both of which have lists of |
| // in-package test files we'll call A' and B', and which have the |
| // following import graph edges: |
| // B imports A |
| // B' imports A |
| // A' imports B |
| // This last edge would be expected to create an error were it not |
| // for the special type-checking discipline above. |
| // Cycles of size greater than two are possible. For example: |
| // compress/bzip2/bzip2_test.go (package bzip2) imports "io/ioutil" |
| // io/ioutil/tempfile_test.go (package ioutil) imports "regexp" |
| // regexp/exec_test.go (package regexp) imports "compress/bzip2" |
| // |
| // |
| // Concurrency |
| // ----------- |
| // |
| // Let us define the import dependency graph as follows. Each node is a |
| // list of files passed to (Checker).Files at once. Many of these lists |
| // are the production code of an importable Go package, so those nodes |
| // are labelled by the package's path. The remaining nodes are |
| // ad hoc packages and lists of in-package *_test.go files that augment |
| // an importable package; those nodes have no label. |
| // |
| // The edges of the graph represent import statements appearing within a |
| // file. An edge connects a node (a list of files) to the node it |
| // imports, which is importable and thus always labelled. |
| // |
| // Loading is controlled by this dependency graph. |
| // |
| // To reduce I/O latency, we start loading a package's dependencies |
| // asynchronously as soon as we've parsed its files and enumerated its |
| // imports (scanImports). This performs a preorder traversal of the |
| // import dependency graph. |
| // |
| // To exploit hardware parallelism, we type-check unrelated packages in |
| // parallel, where "unrelated" means not ordered by the partial order of |
| // the import dependency graph. |
| // |
| // We use a concurrency-safe non-blocking cache (importer.imported) to |
| // record the results of type-checking, whether success or failure. An |
| // entry is created in this cache by startLoad the first time the |
| // package is imported. The first goroutine to request an entry becomes |
| // responsible for completing the task and broadcasting completion to |
| // subsequent requestors, which block until then. |
| // |
| // Type checking occurs in (parallel) postorder: we cannot type-check a |
| // set of files until we have loaded and type-checked all of their |
| // immediate dependencies (and thus all of their transitive |
| // dependencies). If the input were guaranteed free of import cycles, |
| // this would be trivial: we could simply wait for completion of the |
| // dependencies and then invoke the typechecker. |
| // |
| // But as we saw in the 'go test' section above, some cycles in the |
| // import graph over packages are actually legal, so long as the |
| // cycle-forming edge originates in the in-package test files that |
| // augment the package. This explains why the nodes of the import |
| // dependency graph are not packages, but lists of files: the unlabelled |
| // nodes avoid the cycles. Consider packages A and B where B imports A |
| // and A's in-package tests AT import B. The naively constructed import |
| // graph over packages would contain a cycle (A+AT) --> B --> (A+AT) but |
| // the graph over lists of files is AT --> B --> A, where AT is an |
| // unlabelled node. |
| // |
| // Awaiting completion of the dependencies in a cyclic graph would |
| // deadlock, so we must materialize the import dependency graph (as |
| // importer.graph) and check whether each import edge forms a cycle. If |
| // x imports y, and the graph already contains a path from y to x, then |
| // there is an import cycle, in which case the processing of x must not |
| // wait for the completion of processing of y. |
| // |
| // When the type-checker makes a callback (doImport) to the loader for a |
| // given import edge, there are two possible cases. In the normal case, |
| // the dependency has already been completely type-checked; doImport |
| // does a cache lookup and returns it. In the cyclic case, the entry in |
| // the cache is still necessarily incomplete, indicating a cycle. We |
| // perform the cycle check again to obtain the error message, and return |
| // the error. |
| // |
| // The result of using concurrency is about a 2.5x speedup for stdlib_test. |
| |
| // TODO(adonovan): overhaul the package documentation. |
