Revert "cmd/go: remove support for -buildmode=shared"

This reverts CL 359096.

Updates #47788.

Reason for revert: -buildmode=shared may have actually been working in a few very specific cases. We should not remove -buildmode=shared until we have implemented an alternative to support those few cases.

Change-Id: Ia962b06abacc11f6f29fc29d092773be175e32f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/359575
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

8 files changed, 60 insertions, 23 deletions

diff --git a/src/cmd/dist/test.go b/src/cmd/dist/test.go
index 87238cc74f..5935011e19 100644
--- a/src/cmd/dist/test.go
+++ b/src/cmd/dist/test.go
@@ -762,6 +762,9 @@ func (t *tester) registerTests() {
 		if t.supportedBuildmode("c-shared") {
 			t.registerHostTest("testcshared", "../misc/cgo/testcshared", "misc/cgo/testcshared", ".")
 		}
+		if t.supportedBuildmode("shared") {
+			t.registerTest("testshared", "../misc/cgo/testshared", t.goTest(), t.timeout(600), ".")
+		}
 		if t.supportedBuildmode("plugin") {
 			t.registerTest("testplugin", "../misc/cgo/testplugin", t.goTest(), t.timeout(600), ".")
 		}
@@ -1044,6 +1047,12 @@ func (t *tester) supportedBuildmode(mode string) bool {
 			return true
 		}
 		return false
+	case "shared":
+		switch pair {
+		case "linux-386", "linux-amd64", "linux-arm", "linux-arm64", "linux-ppc64le", "linux-s390x":
+			return true
+		}
+		return false
 	case "plugin":
 		// linux-arm64 is missing because it causes the external linker
 		// to crash, see https://golang.org/issue/17138
diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index 1b9b22a812..487a8f580b 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -162,6 +162,9 @@
 // 		flags has a similar effect.
 // 	-ldflags '[pattern=]arg list'
 // 		arguments to pass on each go tool link invocation.
+// 	-linkshared
+// 		build code that will be linked against shared libraries previously
+// 		created with -buildmode=shared.
 // 	-mod mode
 // 		module download mode to use: readonly, vendor, or mod.
 // 		By default, if a vendor directory is present and the go version in go.mod
@@ -779,6 +782,7 @@
 //         Name          string   // package name
 //         Doc           string   // package documentation string
 //         Target        string   // install path
+//         Shlib         string   // the shared library that contains this package (only set when -linkshared)
 //         Goroot        bool     // is this package in the Go root?
 //         Standard      bool     // is this package part of the standard Go library?
 //         Stale         bool     // would 'go install' do anything for this package?
@@ -1799,6 +1803,11 @@
 // 		non-main packages are built into .a files (the default
 // 		behavior).
 //
+// 	-buildmode=shared
+// 		Combine all the listed non-main packages into a single shared
+// 		library that will be used when building with the -linkshared
+// 		option. Packages named main are ignored.
+//
 // 	-buildmode=exe
 // 		Build the listed main packages and everything they import into
 // 		executables. Packages not named main are ignored.
diff --git a/src/cmd/go/internal/help/helpdoc.go b/src/cmd/go/internal/help/helpdoc.go
index 2bb3584eba..035235fe1b 100644
--- a/src/cmd/go/internal/help/helpdoc.go
+++ b/src/cmd/go/internal/help/helpdoc.go
@@ -726,6 +726,11 @@ are:
 		non-main packages are built into .a files (the default
 		behavior).
 
+	-buildmode=shared
+		Combine all the listed non-main packages into a single shared
+		library that will be used when building with the -linkshared
+		option. Packages named main are ignored.
+
 	-buildmode=exe
 		Build the listed main packages and everything they import into
 		executables. Packages not named main are ignored.
diff --git a/src/cmd/go/internal/list/list.go b/src/cmd/go/internal/list/list.go
index f23fbe5fea..8c85ddcf21 100644
--- a/src/cmd/go/internal/list/list.go
+++ b/src/cmd/go/internal/list/list.go
@@ -23,8 +23,8 @@ import (
 	"cmd/go/internal/load"
 	"cmd/go/internal/modinfo"
 	"cmd/go/internal/modload"
-	"cmd/go/internal/str"
 	"cmd/go/internal/work"
+	"cmd/go/internal/str"
 )
 
 var CmdList = &base.Command{
@@ -56,6 +56,7 @@ to -f '{{.ImportPath}}'. The struct being passed to the template is:
         Name          string   // package name
         Doc           string   // package documentation string
         Target        string   // install path
+        Shlib         string   // the shared library that contains this package (only set when -linkshared)
         Goroot        bool     // is this package in the Go root?
         Standard      bool     // is this package part of the standard Go library?
         Stale         bool     // would 'go install' do anything for this package?
diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index e39ffcbd50..9d0ad27f0d 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -116,6 +116,9 @@ and test commands:
 		flags has a similar effect.
 	-ldflags '[pattern=]arg list'
 		arguments to pass on each go tool link invocation.
+	-linkshared
+		build code that will be linked against shared libraries previously
+		created with -buildmode=shared.
 	-mod mode
 		module download mode to use: readonly, vendor, or mod.
 		By default, if a vendor directory is present and the go version in go.mod
diff --git a/src/cmd/go/internal/work/init.go b/src/cmd/go/internal/work/init.go
index 9111150233..dc368de1c1 100644
--- a/src/cmd/go/internal/work/init.go
+++ b/src/cmd/go/internal/work/init.go
@@ -233,20 +233,16 @@ func buildModeInit() {
 		}
 		ldBuildmode = "pie"
 	case "shared":
-		if cfg.Goos == "linux" {
-			switch cfg.Goarch {
-			case "386", "amd64", "arm", "arm64", "ppc64le", "s390x":
-				// -buildmode=shared was supported on these platforms at one point, but
-				// never really worked in module mode.
-				// Support was officially dropped as of Go 1.18.
-				// (See https://golang.org/issue/47788.)
-				base.Fatalf("-buildmode=shared no longer supported as of Go 1.18")
-
-				// TODO(#47788): Remove supporting code for -buildmode=shared.
-				// (For the Go 1.18 release, we will keep most of the code around but
-				// disabled to avoid merge conflicts in case we need to revert quickly.)
-			}
+		pkgsFilter = pkgsNotMain
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			codegenArg = "-dynlink"
+		}
+		if cfg.BuildO != "" {
+			base.Fatalf("-buildmode=shared and -o not supported together")
 		}
+		ldBuildmode = "shared"
 	case "plugin":
 		pkgsFilter = oneMainPkg
 		if gccgo {
@@ -265,15 +261,6 @@ func buildModeInit() {
 	}
 
 	if cfg.BuildLinkshared {
-		if cfg.Goos == "linux" {
-			switch cfg.Goarch {
-			case "386", "amd64", "arm", "arm64", "ppc64le", "s390x":
-				base.Fatalf("-linkshared no longer supported as of Go 1.18")
-				// TODO(#47788): Remove supporting code for linkshared.
-				// (For the Go 1.18 release, we will keep most of the code around but
-				// disabled to avoid merge conflicts in case we need to revert quickly.)
-			}
-		}
 		if !sys.BuildModeSupported(cfg.BuildToolchainName, "shared", cfg.Goos, cfg.Goarch) {
 			base.Fatalf("-linkshared not supported on %s/%s\n", cfg.Goos, cfg.Goarch)
 		}
diff --git a/src/cmd/go/testdata/script/list_linkshared.txt b/src/cmd/go/testdata/script/list_linkshared.txt
new file mode 100644
index 0000000000..baae1e2be8
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_linkshared.txt
@@ -0,0 +1,16 @@
+env GO111MODULE=on
+
+# golang.org/issue/35759: 'go list -linkshared'
+# panicked if invoked on a test-only package.
+
+[!buildmode:shared] skip
+
+go list -f '{{.ImportPath}}: {{.Target}} {{.Shlib}}' -linkshared .
+stdout '^example.com:  $'
+
+-- go.mod --
+module example.com
+
+go 1.14
+-- x.go --
+package x
diff --git a/src/cmd/internal/sys/supported.go b/src/cmd/internal/sys/supported.go
index 18ca50f927..4fa5aa495e 100644
--- a/src/cmd/internal/sys/supported.go
+++ b/src/cmd/internal/sys/supported.go
@@ -132,6 +132,13 @@ func BuildModeSupported(compiler, buildmode, goos, goarch string) bool {
 		}
 		return false
 
+	case "shared":
+		switch platform {
+		case "linux/386", "linux/amd64", "linux/arm", "linux/arm64", "linux/ppc64le", "linux/s390x":
+			return true
+		}
+		return false
+
 	case "plugin":
 		switch platform {
 		case "linux/amd64", "linux/arm", "linux/arm64", "linux/386", "linux/s390x", "linux/ppc64le",