go/build: change the search order for "vendor/" paths based on srcDir

If srcDir is within GOROOT, prefer GOROOT.
Otherwise, prefer GOPATH.

The attached tests may seem a bit strange; they will make more sense
in a followup CL.

Updates #16333
Updates #30241
Updates #30228

Change-Id: Ic5f1334cce5e242d7f49080aba083bcf2080dee3
Reviewed-on: https://go-review.googlesource.com/c/go/+/164619
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/testdata/script/gopath_std_vendor.txt

+env GO111MODULE=off
+
+[!gc] skip
+
+# A package importing 'net/http' should resolve its dependencies
+# to the package 'vendor/golang.org/x/net/http2/hpack' within GOROOT.
+cd importnethttp
+go list -deps -f '{{.ImportPath}} {{.Dir}}'
+stdout ^internal/x/net/http2/hpack
+stdout $GOROOT[/\\]src[/\\]internal[/\\]x[/\\]net[/\\]http2[/\\]hpack
+! stdout $GOPATH[/\\]src[/\\]vendor
+
+# In the presence of $GOPATH/src/vendor/golang.org/x/net/http2/hpack,
+# a package in GOPATH importing 'golang.org/x/net/http2/hpack' should
+# resolve its dependencies in GOPATH/src.
+cd ../issue16333
+go build .
+
+go list -deps -f '{{.ImportPath}} {{.Dir}}' .
+stdout $GOPATH[/\\]src[/\\]vendor[/\\]golang.org[/\\]x[/\\]net[/\\]http2[/\\]hpack
+! stdout $GOROOT[/\\]src[/\\]vendor
+
+go list -test -deps -f '{{.ImportPath}} {{.Dir}}' .
+stdout $GOPATH[/\\]src[/\\]vendor[/\\]golang.org[/\\]x[/\\]net[/\\]http2[/\\]hpack
+! stdout $GOROOT[/\\]src[/\\]vendor
+
+-- issue16333/issue16333.go --
+package vendoring17
+
+import _ "golang.org/x/net/http2/hpack"
+-- issue16333/issue16333_test.go --
+package vendoring17
+
+import _ "testing"
+import _ "golang.org/x/net/http2/hpack"
+-- importnethttp/http.go --
+package importnethttp
+
+import _ "net/http"
+-- $GOPATH/src/vendor/golang.org/x/net/http2/hpack/hpack.go --
+package hpack

src/go/build/build.go

@@ -647,18 +647,28 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 
 		// Determine directory from import path.
 		if ctxt.GOROOT != "" {
-			dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
-			if ctxt.Compiler != "gccgo" {
-				isDir := ctxt.isDir(dir)
-				binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
-				if isDir || binaryOnly {
-					p.Dir = dir
-					p.Goroot = true
-					p.Root = ctxt.GOROOT
-					goto Found
+			// If the package path starts with "vendor/", only search GOROOT before
+			// GOPATH if the importer is also within GOROOT. That way, if the user has
+			// vendored in a package that is subsequently included in the standard
+			// distribution, they'll continue to pick up their own vendored copy.
+			gorootFirst := srcDir == "" || !strings.HasPrefix(path, "vendor/")
+			if !gorootFirst {
+				_, gorootFirst = ctxt.hasSubdir(ctxt.GOROOT, srcDir)
+			}
+			if gorootFirst {
+				dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+				if ctxt.Compiler != "gccgo" {
+					isDir := ctxt.isDir(dir)
+					binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
+					if isDir || binaryOnly {
+						p.Dir = dir
+						p.Goroot = true
+						p.Root = ctxt.GOROOT
+						goto Found
+					}
 				}
+				tried.goroot = dir
 			}
-			tried.goroot = dir
 		}
 		if ctxt.Compiler == "gccgo" && goroot.IsStandardPackage(ctxt.GOROOT, ctxt.Compiler, path) {
 			p.Dir = ctxt.joinPath(ctxt.GOROOT, "src", path)
@@ -678,6 +688,24 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 			tried.gopath = append(tried.gopath, dir)
 		}
 
+		// If we tried GOPATH first due to a "vendor/" prefix, fall back to GOPATH.
+		// That way, the user can still get useful results from 'go list' for
+		// standard-vendored paths passed on the command line.
+		if ctxt.GOROOT != "" && tried.goroot == "" {
+			dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+			if ctxt.Compiler != "gccgo" {
+				isDir := ctxt.isDir(dir)
+				binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
+				if isDir || binaryOnly {
+					p.Dir = dir
+					p.Goroot = true
+					p.Root = ctxt.GOROOT
+					goto Found
+				}
+			}
+			tried.goroot = dir
+		}
+
 		// package was not found
 		var paths []string
 		format := "\t%s (vendor tree)"