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+// 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.
+//
+// Deprecated: This is an older API and does not have support
+// for modules. Use golang.org/x/tools/go/packages instead.
+//
+// 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.
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