go/build: invoke go command to find modules during Import, Context.Import

The introduction of modules has broken (intentionally) the rule
that the source code for a package x/y/z is in GOPATH/src/x/y/z
(or GOROOT/src/x/y/z). This breaks the code in go/build.Import,
which uses that rule to find the directory for a package.

In the long term, the fix is to move programs that load packages
off of go/build and onto golang.org/x/tools/go/packages, which
we hope will eventually become go/packages. That code invokes
the go command to learn what it needs to know about where
packages are.

In the short term, though, there are lots of programs that use go/build
and will not be able to find code in module dependencies.
To help those programs, go/build now runs the go command to
ask where a package's source code can be found, if it sees that
modules are in use. (If modules are not in use, it falls back to the
usual lookup code and does not invoke the go command, so that
existing uses are unaffected and not slowed down.)

Helps #24661.
Fixes #26504.

Change-Id: I0dac68854cf5011005c3b2272810245d81b7cc5a
Reviewed-on: https://go-review.googlesource.com/125296
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/src/cmd/go/internal/cfg/cfg.go b/src/cmd/go/internal/cfg/cfg.go
index 9dd90ee871..c7746b6912 100644
--- a/src/cmd/go/internal/cfg/cfg.go
+++ b/src/cmd/go/internal/cfg/cfg.go
@@ -20,7 +20,7 @@ import (
 var (
 	BuildA                 bool   // -a flag
 	BuildBuildmode         string // -buildmode flag
-	BuildContext           = build.Default
+	BuildContext           = defaultContext()
 	BuildGetmode           string             // -getmode flag
 	BuildI                 bool               // -i flag
 	BuildLinkshared        bool               // -linkshared flag
@@ -43,6 +43,12 @@ var (
 	DebugActiongraph string // -debug-actiongraph flag (undocumented, unstable)
 )
 
+func defaultContext() build.Context {
+	ctxt := build.Default
+	ctxt.JoinPath = filepath.Join // back door to say "do not use go command"
+	return ctxt
+}
+
 func init() {
 	BuildToolchainCompiler = func() string { return "missing-compiler" }
 	BuildToolchainLinker = func() string { return "missing-linker" }