cmd/go: add GOPRIVATE environment variable

It is too confusing to have to set GONOSUMDB and GONOPROXY
in common use cases, but one cannot be guaranteed to be a
subset of the other.

This CL adds GOPRIVATE, which takes the same kind of pattern list
but is defined as "these patterns are private (non-public) modules".
Today the implication is that GOPRIVATE is the default setting for
GONOSUMDB and GONOPROXY. If there are other accommodations
to make for private packages in the future or in other tools,
having this clear statement of intent will let us do that.
(For example maybe an IDE integration would hyperlink an import
path to godoc.org; consulting GOPRIVATE would be a reasonable
way to decide not to do that for certain imports. In contrast,
consulting GONOPROXY or GONOSUMDB clearly would not.)

Fixes #32184.

Change-Id: If54c12d353c7a0a5c0e0273764140cce3c154a02
Reviewed-on: https://go-review.googlesource.com/c/go/+/181719
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 47 insertions, 9 deletions

diff --git a/src/cmd/go/internal/modfetch/fetch.go b/src/cmd/go/internal/modfetch/fetch.go
index 94cb0d3a19..bc1d35e690 100644
--- a/src/cmd/go/internal/modfetch/fetch.go
+++ b/src/cmd/go/internal/modfetch/fetch.go
@@ -631,7 +631,7 @@ For more information, see 'go help module-auth'.
 
 `
 
-var HelpSum = &base.Command{
+var HelpModuleAuth = &base.Command{
 	UsageLine: "module-auth",
 	Short:     "module authentication using go.sum",
 	Long: `
@@ -712,18 +712,56 @@ If GOSUMDB is set to "off", or if "go get" is invoked with the -insecure flag,
 the checksum database is not consulted, and all unrecognized modules are
 accepted, at the cost of giving up the security guarantee of verified repeatable
 downloads for all modules. A better way to bypass the checksum database
-for specific modules is to use the GONOSUMDB environment variable.
+for specific modules is to use the GOPRIVATE or GONOSUMDB environment
+variables. See 'go help module-private' for details.
 
-The GONOSUMDB environment variable is a comma-separated list of
-glob patterns (in the syntax of Go's path.Match) of module path prefixes
-that should not be compared against the checksum database.
+The 'go env -w' command (see 'go help env') can be used to set these variables
+for future go command invocations.
+`,
+}
+
+var HelpModulePrivate = &base.Command{
+	UsageLine: "module-private",
+	Short:     "module configuration for non-public modules",
+	Long: `
+The go command defaults to downloading modules from the public Go module
+mirror at proxy.golang.org. It also defaults to validating downloaded modules,
+regardless of source, against the public Go checksum database at sum.golang.org.
+These defaults work well for publicly available source code.
+
+The GOPRIVATE environment variable controls which modules the go command
+considers to be private (not available publicly) and should therefore not use the
+proxy or checksum database. The variable is a comma-separated list of
+glob patterns (in the syntax of Go's path.Match) of module path prefixes.
 For example,
 
-	GONOSUMDB=*.corp.example.com,rsc.io/private
+	GOPRIVATE=*.corp.example.com,rsc.io/private
+
+causes the go command to treat as private any module with a path prefix
+matching either pattern, including git.corp.example.com/xyzzy, rsc.io/private,
+and rsc.io/private/quux.
+
+The GOPRIVATE environment variable may be used by other tools as well to
+identify non-public modules. For example, an editor could use GOPRIVATE
+to decide whether to hyperlink a package import to a godoc.org page.
+
+For fine-grained control over module download and validation, the GONOPROXY
+and GONOSUMDB environment variables accept the same kind of glob list
+and override GOPRIVATE for the specific decision of whether to use the proxy
+and checksum database, respectively.
+
+For example, if a company ran a module proxy serving private modules,
+users would configure go using:
+
+	GOPRIVATE=*.corp.example.com
+	GOPROXY=proxy.example.com
+	GONOPROXY=none
 
-disables checksum database lookups for modules with path prefixes matching
-either pattern, including "git.corp.example.com/xyzzy", "rsc.io/private",
-and "rsc.io/private/quux".
+This would tell the go comamnd and other tools that modules beginning with
+a corp.example.com subdomain are private but that the company proxy should
+be used for downloading both public and private modules, because
+GONOPROXY has been set to a pattern that won't match any modules,
+overriding GOPRIVATE.
 
 The 'go env -w' command (see 'go help env') can be used to set these variables
 for future go command invocations.