libdraw: fix zero stringwidth() bug

stringwidth() and string() sometimes failed spuriously due
to the wrong assumption that cachechars() will only fail
when a different subfont is needed. in fact, cachechars()
can fail for other reasons like when it resizes the fontcache
(or fails todo so).

theres also the case when loadchar() is unable to translate
a character and returns 0. this case needs to be differentiated
from such temporary conditions like fontcache resize or subfont
load to stop the retry loop in string() and stringwidth().

now cachechars() returns -1 to indicate that it cannot
proceed and we test this in string() and stringwidth()
to skip over untranslatable characters to make progress
instead of retrying.

3 files changed, 14 insertions, 12 deletions

diff --git a/sys/src/libdraw/font.c b/sys/src/libdraw/font.c
index 4eaeb9e8d..7247c9a92 100644
--- a/sys/src/libdraw/font.c
+++ b/sys/src/libdraw/font.c
@@ -7,10 +7,11 @@ static int	freeup(Font*);
 
 #define	PJW	0	/* use NUL==pjw for invisible characters */
 
+/* return number of translated cache indices, 0 must retry, -1 on error */
 int
 cachechars(Font *f, char **ss, Rune **rr, ushort *cp, int max, int *wp, char **subfontname)
 {
-	int i, th, sh, h, w, rw, wid, nc;
+	int i, j, th, sh, h, w, rw, wid, nc;
 	char *sp;
 	Rune r, *rp, vr;
 	ulong a;
@@ -78,14 +79,16 @@ cachechars(Font *f, char **ss, Rune **rr, ushort *cp, int max, int *wp, char **s
 			}
 		}
 
-		if(c->age == f->age)	/* flush pending string output */
+		if(i > 0 && c->age == f->age)	/* flush pending string output */
 			break;
 
-		if(loadchar(f, r, c, h, i, subfontname) <= 0)
-			break;
+		j = loadchar(f, r, c, h, i, subfontname);
+		if(j <= 0){
+			if(j < 0 || i > 0)	/* flush output or retry */ 
+				break;
+			return -1;		/* stop retrying */
+		}
 
-		c = &f->cache[h];	/* may have reallocated f->cache */
-	
 	    Found:
 		wid += c->width;
 		c->age = f->age;
@@ -282,9 +285,8 @@ loadchar(Font *f, Rune r, Cacheinfo *c, int h, int noflush, char **subfontname)
 			f->width = wid;
 		if(f->maxdepth < subf->f->bits->depth)
 			f->maxdepth = subf->f->bits->depth;
-		i = fontresize(f, f->width, f->ncache, f->maxdepth);
-		if(i <= 0)
-			return i;
+		if(fontresize(f, f->width, f->ncache, f->maxdepth) <= 0)
+			return -1;
 		/* c is still valid as didn't reallocate f->cache */
 	}
 	c->value = r;
@@ -340,7 +342,7 @@ freeup(Font *f)
 	return nf;
 }
 
-/* return whether resize succeeded && f->cache is unchanged */
+/* returns whether resize succeeded && f->cache is unchanged */
 static int
 fontresize(Font *f, int wid, int ncache, int depth)
 {
diff --git a/sys/src/libdraw/string.c b/sys/src/libdraw/string.c
index 2dc2a63e7..c15bf2777 100644
--- a/sys/src/libdraw/string.c
+++ b/sys/src/libdraw/string.c
@@ -92,7 +92,7 @@ _string(Image *dst, Point pt, Image *src, Point sp, Font *f, char *s, Rune *r, i
 			}
 		}
 		if((n = cachechars(f, sptr, rptr, cbuf, max, &wid, &subfontname)) <= 0){
-			if(subfontname){
+			if(n == 0){
 				if(++try > 10)
 					break;
 				continue;
diff --git a/sys/src/libdraw/stringwidth.c b/sys/src/libdraw/stringwidth.c
index f0757a095..3548aba0b 100644
--- a/sys/src/libdraw/stringwidth.c
+++ b/sys/src/libdraw/stringwidth.c
@@ -39,7 +39,7 @@ _stringnwidth(Font *f, char *s, Rune *r, int len)
 			}
 		}
 		if((n = cachechars(f, sptr, rptr, cbuf, max, &wid, &subfontname)) <= 0){
-			if(subfontname){
+			if(n == 0){
 				if(++try > 10)
 					break;
 				continue;