gc: add -p flag to catch import cycles earlier

The linker would catch them if gc succeeded,
but too often the cycle manifests as making the
current package and the imported copy of itself
appear as different packages, which result in
type signature mismatches that confuse users.

As a crutch, add the -p flag to say 'if you see an
import of this package, give up early'.  Results in
messages like (during gotest in sort):

export_test.go:7: import "sort" while compiling that package (import cycle)
export_test.go:7: import "container/heap": package depends on "sort" (import cycle)

Fixes #2042.

R=ken
CC=bradfitz, dsymonds, golang-dev
https://golang.org/cl/4972057

src/Make.pkg

@@ -83,10 +83,10 @@ $(TARGDIR)/$(TARG).a: _obj/$(TARG).a
 	cp _obj/$(TARG).a "$@"
 
 _go_.$O: $(GOFILES) $(PREREQ)
-	$(GC) $(GCIMPORTS) -o $@ $(GOFILES)
+	$(GC) $(GCIMPORTS) -p $(TARG) -o $@ $(GOFILES)
 
 _gotest_.$O: $(GOFILES) $(GOTESTFILES) $(PREREQ)
-	$(GC) $(GCIMPORTS) -o $@ $(GOFILES) $(GOTESTFILES)
+	$(GC) $(GCIMPORTS) -p $(TARG) -o $@ $(GOFILES) $(GOTESTFILES)
 
 _obj/$(TARG).a: _go_.$O $(OFILES)
 	@mkdir -p _obj/$(dir)

src/cmd/gc/doc.go

@@ -35,6 +35,9 @@ Flags:
 		output file, default file.6 for 6g, etc.
 	-e
 		normally the compiler quits after 10 errors; -e prints all errors
+	-p path
+		assume that path is the eventual import path for this code,
+		and diagnose any attempt to import a package that depends on it.
 	-L
 		show entire file path when printing line numbers in errors
 	-I dir1 -I dir2

src/cmd/gc/go.h

@@ -773,6 +773,7 @@ EXTERN	Pkg*	phash[128];
 EXTERN	int	tptr;		// either TPTR32 or TPTR64
 extern	char*	runtimeimport;
 extern	char*	unsafeimport;
+EXTERN	char*	myimportpath;
 EXTERN	Idir*	idirs;
 
 EXTERN	Type*	types[NTYPE];

src/cmd/gc/go.y

@@ -243,11 +243,11 @@ import_package:
 			importpkg->name = $2->name;
 			pkglookup($2->name, nil)->npkg++;
 		} else if(strcmp(importpkg->name, $2->name) != 0)
-			yyerror("conflicting names %s and %s for package %Z", importpkg->name, $2->name, importpkg->path);
+			yyerror("conflicting names %s and %s for package \"%Z\"", importpkg->name, $2->name, importpkg->path);
 		importpkg->direct = 1;
 		
 		if(safemode && !curio.importsafe)
-			yyerror("cannot import unsafe package %Z", importpkg->path);
+			yyerror("cannot import unsafe package \"%Z\"", importpkg->path);
 	}
 
 import_safety:
@@ -1686,7 +1686,11 @@ hidden_import:
 			p->name = $2->name;
 			pkglookup($2->name, nil)->npkg++;
 		} else if(strcmp(p->name, $2->name) != 0)
-			yyerror("conflicting names %s and %s for package %Z", p->name, $2->name, p->path);
+			yyerror("conflicting names %s and %s for package \"%Z\"", p->name, $2->name, p->path);
+		if(!incannedimport && myimportpath != nil && strcmp($3.u.sval->s, myimportpath) == 0) {
+			yyerror("import \"%Z\": package depends on \"%Z\" (import cycle)", importpkg->path, $3.u.sval);
+			errorexit();
+		}
 	}
 |	LVAR hidden_pkg_importsym hidden_type ';'
 	{

src/cmd/gc/lex.c

@@ -88,6 +88,7 @@ usage(void)
 	print("  -h panic on an error\n");
 	print("  -m print about moves to heap\n");
 	print("  -o file specify output file\n");
+	print("  -p assumed import path for this code\n");
 	print("  -s disable escape analysis\n");
 	print("  -u disable package unsafe\n");
 	print("  -w print the parse tree after typing\n");
@@ -154,6 +155,10 @@ main(int argc, char *argv[])
 	case 'o':
 		outfile = EARGF(usage());
 		break;
+	
+	case 'p':
+		myimportpath = EARGF(usage());
+		break;
 
 	case 'I':
 		addidir(EARGF(usage()));
@@ -479,6 +484,11 @@ importfile(Val *f, int line)
 		errorexit();
 	}
 
+	if(myimportpath != nil && strcmp(f->u.sval->s, myimportpath) == 0) {
+		yyerror("import \"%Z\" while compiling that package (import cycle)", f->u.sval);
+		errorexit();
+	}
+
 	if(strcmp(f->u.sval->s, "unsafe") == 0) {
 		if(safemode) {
 			yyerror("cannot import package unsafe");
@@ -500,14 +510,14 @@ importfile(Val *f, int line)
 	}
 
 	if(!findpkg(path)) {
-		yyerror("can't find import: %Z", f->u.sval);
+		yyerror("can't find import: \"%Z\"", f->u.sval);
 		errorexit();
 	}
 	importpkg = mkpkg(path);
 
 	imp = Bopen(namebuf, OREAD);
 	if(imp == nil) {
-		yyerror("can't open import: %Z: %r", f->u.sval);
+		yyerror("can't open import: \"%Z\": %r", f->u.sval);
 		errorexit();
 	}
 	file = strdup(namebuf);
@@ -564,7 +574,7 @@ importfile(Val *f, int line)
 			continue;
 		return;
 	}
-	yyerror("no import in: %Z", f->u.sval);
+	yyerror("no import in \"%Z\"", f->u.sval);
 	unimportfile();
 }
 
@@ -1938,7 +1948,7 @@ mkpackage(char* pkgname)
 					// errors if a conflicting top-level name is
 					// introduced by a different file.
 					if(!s->def->used && !nsyntaxerrors)
-						yyerrorl(s->def->lineno, "imported and not used: %Z", s->def->pkg->path);
+						yyerrorl(s->def->lineno, "imported and not used: \"%Z\"", s->def->pkg->path);
 					s->def = N;
 					continue;
 				}
@@ -1946,7 +1956,7 @@ mkpackage(char* pkgname)
 					// throw away top-level name left over
 					// from previous import . "x"
 					if(s->def->pack != N && !s->def->pack->used && !nsyntaxerrors) {
-						yyerrorl(s->def->pack->lineno, "imported and not used: %Z", s->def->pack->pkg->path);
+						yyerrorl(s->def->pack->lineno, "imported and not used: \"%Z\"", s->def->pack->pkg->path);
 						s->def->pack->used = 1;
 					}
 					s->def = N;

src/cmd/gc/subr.c

@@ -405,7 +405,7 @@ importdot(Pkg *opkg, Node *pack)
 	}
 	if(n == 0) {
 		// can't possibly be used - there were no symbols
-		yyerrorl(pack->lineno, "imported and not used: %Z", opkg->path);
+		yyerrorl(pack->lineno, "imported and not used: \"%Z\"", opkg->path);
 	}
 }