cmd/go: document $GOPROXY, other module adjustments

Also document module use of GOPATH including GOPATH/src/mod
and GOPATH/bin (unless GOBIN is set).

Fixes #26399.
Fixes #26406.

Change-Id: I7be8eaf110f4fa6fc76ea4cd39aea3dd8addf0b0
Reviewed-on: https://go-review.googlesource.com/124707Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/help/helpdoc.go

@@ -195,6 +195,7 @@ using the named version control system, and then the path inside
 that repository. The supported version control systems are:
 
 	Bazaar      .bzr
+	Fossil      .fossil
 	Git         .git
 	Mercurial   .hg
 	Subversion  .svn
@@ -238,7 +239,7 @@ The meta tag should appear as early in the file as possible.
 In particular, it should appear before any raw JavaScript or CSS,
 to avoid confusing the go command's restricted parser.
 
-The vcs is one of "git", "hg", "svn", etc,
+The vcs is one of "bzr", "fossil", "git", "hg", "svn".
 
 The repo-root is the root of the version control system
 containing a scheme and not containing a .vcs qualifier.
@@ -260,12 +261,22 @@ the go tool will verify that https://example.org/?go-get=1 contains the
 same meta tag and then git clone https://code.org/r/p/exproj into
 GOPATH/src/example.org.
 
-New downloaded packages are written to the first directory listed in the GOPATH
-environment variable (For more details see: 'go help gopath').
+When using GOPATH, downloaded packages are written to the first directory
+listed in the GOPATH environment variable.
+(See 'go help gopath-get' and 'go help gopath'.)
+
+When using modules, downloaded packages are stored in the module cache.
+(See 'go help modules-get' and 'go help goproxy'.)
+
+When using modules, an additional variant of the go-import meta tag is
+recognized and is preferred over those listing version control systems.
+That variant uses "mod" as the vcs in the content value, as in:
+
+	<meta name="go-import" content="example.org mod https://code.org/moduleproxy">
 
-The go command attempts to download the version of the
-package appropriate for the Go release being used.
-Run 'go help get' for more.
+This tag means to fetch modules with paths beginning with example.org
+from the module proxy available at the URL https://code.org/moduleproxy.
+See 'go help goproxy' for details about the proxy protocol.
 
 Import path checking
 
@@ -288,6 +299,9 @@ Import path checking is disabled for code found within vendor trees.
 This makes it possible to copy code into alternate locations in vendor trees
 without needing to update import comments.
 
+Import path checking is also disabled when using modules.
+Import path comments are obsoleted by the go.mod file's module statement.
+
 See https://golang.org/s/go14customimport for details.
 	`,
 }
@@ -360,6 +374,12 @@ in the list.
 
 See https://golang.org/doc/code.html for an example.
 
+GOPATH and Modules
+
+When using modules, GOPATH is no longer used for resolving imports.
+However, it is still used to store downloaded source code (in GOPATH/src/mod)
+and compiled commands (in GOPATH/bin).
+
 Internal Directories
 
 Code in or below a directory named "internal" is importable only
@@ -471,6 +491,8 @@ General-purpose environment variables:
 		Examples are linux, darwin, windows, netbsd.
 	GOPATH
 		For more details see: 'go help gopath'.
+	GOPROXY
+		URL of Go module proxy. See 'go help goproxy'.
 	GORACE
 		Options for the race detector.
 		See https://golang.org/doc/articles/race_detector.html.

src/cmd/go/internal/modfetch/proxy.go

@@ -14,18 +14,85 @@ import (
 	"strings"
 	"time"
 
+	"cmd/go/internal/base"
 	"cmd/go/internal/modfetch/codehost"
 	"cmd/go/internal/module"
 	"cmd/go/internal/semver"
 )
 
+var HelpGoproxy = &base.Command{
+	UsageLine: "goproxy",
+	Short:     "module proxy protocol",
+	Long: `
+The go command by default downloads modules from version control systems
+directly, just as 'go get' always has. If the GOPROXY environment variable
+is set to the URL of a module proxy, the go command will instead fetch
+all modules from that proxy. No matter the source of the modules, downloaded
+modules must match existing entries in go.sum (see 'go help modules' for
+discussion of verification).
+
+A Go module proxy is any web server that can respond to GET requests for
+URLs of a specified form. The requests have no query parameters, so even
+a site serving from a fixed file system (including a file:/// URL)
+can be a module proxy.
+
+The GET requests sent to a Go module proxy are:
+
+GET $GOPROXY/<module>/@v/list returns a list of all known versions of the
+given module, one per line.
+
+GET $GOPROXY/<module>/@v/<version>.info returns JSON-formatted metadata
+about that version of the given module.
+
+GET $GOPROXY/<module>/@v/<version>.mod returns the go.mod file
+for that version of the given module.
+
+GET $GOPROXY/<module>/@v/<version>.zip returns the zip archive
+for that version of the given module.
+
+To avoid problems when serving from case-sensitive file systems,
+the <module> and <version> elements are case-encoded, replacing every
+uppercase letter with an exclamation mark followed by the correponding
+lower-case letter: github.com/Azure encodes as github.com/!azure.
+
+The JSON-formatted metadata about a given module corresponds to
+this Go data structure, which may be expanded in the future:
+
+    type Info struct {
+        Version string    // version string
+        Time    time.Time // commit time
+    }
+
+The zip archive for a specific version of a given module is a
+standard zip file that contains the file tree corresponding
+to the module's source code and related files. The archive uses
+slash-separated paths, and every file path in the archive must
+begin with <module>@<version>/, where the module and version are
+substituted directly, not case-encoded. The root of the module
+file tree corresponds to the <module>@<version>/ prefix in the
+archive.
+
+Even when downloading directly from version control systems,
+the go command synthesizes explicit info, mod, and zip files
+and stores them in its local cache, $GOPATH/src/mod/cache/download,
+the same as if it had downloaded them directly from a proxy.
+The cache layout is the same as the proxy URL space, so
+serving $GOPATH/src/mod/cache/download at (or copying it to)
+https://example.com/proxy would let other users access those
+cached module versions with GOPROXY=https://example.com/proxy.
+`,
+}
+
 var proxyURL = os.Getenv("GOPROXY")
 
 func lookupProxy(path string) (Repo, error) {
+	if strings.Contains(proxyURL, ",") {
+		return nil, fmt.Errorf("invalid $GOPROXY setting: cannot have comma")
+	}
 	u, err := url.Parse(proxyURL)
 	if err != nil || u.Scheme != "http" && u.Scheme != "https" && u.Scheme != "file" {
 		// Don't echo $GOPROXY back in case it has user:password in it (sigh).
-		return nil, fmt.Errorf("invalid $GOPROXY setting")
+		return nil, fmt.Errorf("invalid $GOPROXY setting: malformed URL or invalid scheme (must be http, https, file)")
 	}
 	return newProxyRepo(u.String(), path)
 }

src/cmd/go/internal/modload/help.go

@@ -49,6 +49,10 @@ Module support is enabled only when the current directory is outside
 GOPATH/src and itself contains a go.mod file or is below a directory
 containing a go.mod file.
 
+In module-aware mode, GOPATH no longer defines the meaning of imports
+during a build, but it still stores downloaded dependencies (in GOPATH/src/mod)
+and installed commands (in GOPATH/bin, unless GOBIN is set).
+
 Defining a module
 
 A module is defined by a tree of Go source files with a go.mod file
@@ -245,7 +249,6 @@ For example, these commands are all valid:
 	go get github.com/gorilla/mux@c856192   # records v0.0.0-20180517173623-c85619274f5d
 	go get github.com/gorilla/mux@master    # records current meaning of master
 
-
 Module compatibility and semantic versioning
 
 The go command requires that modules use semantic versions and expects that
@@ -314,7 +317,15 @@ See https://research.swtch.com/vgo-import for more information about
 semantic import versioning, and see https://semver.org/ for more about
 semantic versioning.
 
-Module verification
+Module code layout
+
+For now, see https://research.swtch.com/vgo-module for information
+about how source code in version control systems is mapped to
+module file trees.
+
+TODO: Add documentation to go command.
+
+Module downloading and verification
 
 The go command maintains, in the main module's root directory alongside
 go.mod, a file named go.sum containing the expected cryptographic checksums
@@ -330,6 +341,13 @@ each command invocation. The 'go mod -verify' command checks that
 the cached copies of module downloads still match both their recorded
 checksums and the entries in go.sum.
 
+The go command can fetch modules from a proxy instead of connecting
+to source control systems directly, according to the setting of the GOPROXY
+environment variable.
+
+See 'go help goproxy' for details about the proxy and also the format of
+the cached downloaded packages.
+
 Modules and vendoring
 
 When using modules, the go command completely ignores vendor directories.