ld: do not load the same object file multiple times.

eliminates spurious multiple initialization errors.

give more information in the multiple init errors that remain.

Fixes #87.

R=r
CC=golang-dev
https://golang.org/cl/194052

src/cmd/5l/obj.c

@@ -659,6 +659,12 @@ loop:
 		if(s != S) {
 			p->dlink = s->data;
 			s->data = p;
+			if(s->file == nil)
+				s->file = pn;
+			else if(s->file != pn) {
+				diag("multiple initialization for %s: in both %s and %s", s->name, s->file, pn);
+				errorexit();
+			}			
 		}
 		if(edatap == P)
 			datap = p;

src/cmd/6l/obj.c

@@ -692,6 +692,12 @@ loop:
 		if(s != S) {
 			p->dlink = s->data;
 			s->data = p;
+			if(s->file == nil)
+				s->file = pn;
+			else if(s->file != pn) {
+				diag("multiple initialization for %s: in both %s and %s", s->name, s->file, pn);
+				errorexit();
+			}
 		}
 		if(edatap == P)
 			datap = p;

src/cmd/8l/obj.c

@@ -729,6 +729,12 @@ loop:
 		if(s != S) {
 			p->dlink = s->data;
 			s->data = p;
+			if(s->file == nil)
+				s->file = pn;
+			else if(s->file != pn) {
+				diag("multiple initialization for %s: in both %s and %s", s->name, s->file, pn);
+				errorexit();
+			}			
 		}
 		if(edatap == P)
 			datap = p;

src/cmd/ld/lib.c

@@ -338,13 +338,21 @@ ldobj(Biobuf *f, char *pkg, int64 len, char *pn)
 	static int files;
 	static char **filen;
 	char **nfilen, *line;
-	int n, c1, c2, c3;
+	int i, n, c1, c2, c3;
 	vlong import0, import1, eof;
 	char src[1024];
 
 	eof = Boffset(f) + len;
 	src[0] = '\0';
 
+	// don't load individual object more than once.
+	// happens with import of .6 files because of loop in xresolv.
+	// doesn't happen with .a because SYMDEF is consulted
+	// first to decide whether each individual object file is needed.
+	for(i=0; i<files; i++)
+		if(strcmp(filen[i], pn) == 0)
+			return;
+
 	if((files&15) == 0){
 		nfilen = malloc((files+16)*sizeof(char*));
 		memmove(nfilen, filen, files*sizeof(char*));
@@ -354,7 +362,6 @@ ldobj(Biobuf *f, char *pkg, int64 len, char *pn)
 	pn = strdup(pn);
 	filen[files++] = pn;
 
-
 	/* check the header */
 	line = Brdline(f, '\n');
 	if(line == nil) {
@@ -390,7 +397,6 @@ ldobj(Biobuf *f, char *pkg, int64 len, char *pn)
 	ldpkg(f, pkg, import1 - import0 - 2, pn);	// -2 for !\n
 	Bseek(f, import1, 0);
 
-	// PGNS: Should be using import path, not pkg.
 	ldobj1(f, pkg, eof - Boffset(f), pn);
 	return;