cmd/link: link dynamic library automatically

cgo_import_dynamic pragma indicates a symbol is imported from a
dynamic library. Currently, the linker does not actually link
against the dynamic library, so we have to "force" it by using

//go:cgo_import_dynamic _ _ "dylib"

syntax, which links in the library unconditionally.

This CL changes it to link in the library automatically when a
symbol is imported from the library, without using the "force"
syntax. (The "force" syntax is still supported.)

Remove the unconditional imports in the runtime. Now,
Security.framework and CoreFoundation.framework are only linked
when the x509 package is imported (or otherwise specified).

Fixes #40727.

Change-Id: Ied36b1f621cdcc5dc4a8f497cdf1c554a182d0e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/248333
Run-TryBot: Cherry Zhang <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: cherrymui

src/cmd/link/internal/ld/go.go

@@ -183,6 +183,9 @@ func setCgoAttr(ctxt *Link, lookup func(string, int) loader.Sym, file string, pk
 					hostObjSyms[s] = struct{}{}
 				}
 				havedynamic = 1
+				if lib != "" && ctxt.IsDarwin() {
+					machoadddynlib(lib, ctxt.LinkMode)
+				}
 			}
 
 			continue

src/runtime/sys_darwin.go

@@ -489,9 +489,3 @@ func setNonblock(fd int32) {
 //go:cgo_import_dynamic libc_pthread_cond_wait pthread_cond_wait "/usr/lib/libSystem.B.dylib"
 //go:cgo_import_dynamic libc_pthread_cond_timedwait_relative_np pthread_cond_timedwait_relative_np "/usr/lib/libSystem.B.dylib"
 //go:cgo_import_dynamic libc_pthread_cond_signal pthread_cond_signal "/usr/lib/libSystem.B.dylib"
-
-// Magic incantation to get libSystem and friends actually dynamically linked.
-// TODO: Why does the code require this?  See cmd/link/internal/ld/go.go
-//go:cgo_import_dynamic _ _ "/usr/lib/libSystem.B.dylib"
-//go:cgo_import_dynamic _ _ "/System/Library/Frameworks/Security.framework/Versions/A/Security"
-//go:cgo_import_dynamic _ _ "/System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation"