improve json escape decoder, add test suite

test/json.c

@@ -0,0 +1,32 @@
+#include <assert.h>
+#include <string.h>
+#include <textcode.h>
+
+int main() {
+  char buf[100];
+  size_t l;
+  assert(scan_jsonescape("fnord",buf,&l)==5 && l==5 && !memcmp(buf,"fnord",5));
+  /* is \n properly unescaped? */
+  assert(scan_jsonescape("a\\nb",buf,&l)==4 && l==3 && !memcmp(buf,"a\nb",3));
+  assert(scan_jsonescape("a\\rb",buf,&l)==4 && l==3 && !memcmp(buf,"a\rb",3));
+  assert(scan_jsonescape("a\\bb",buf,&l)==4 && l==3 && !memcmp(buf,"a\bb",3));
+  assert(scan_jsonescape("a\\fb",buf,&l)==4 && l==3 && !memcmp(buf,"a\fb",3));
+  assert(scan_jsonescape("a\\tb",buf,&l)==4 && l==3 && !memcmp(buf,"a\tb",3));
+  assert(scan_jsonescape("a\\\\b",buf,&l)==4 && l==3 && !memcmp(buf,"a\\b",3));
+  assert(scan_jsonescape("a\\/b",buf,&l)==4 && l==3 && !memcmp(buf,"a/b",3));
+  assert(scan_jsonescape("a\\\"b",buf,&l)==4 && l==3 && !memcmp(buf,"a\"b",3));
+  /* does a double quote end the string? */
+  assert(scan_jsonescape("a\"b",buf,&l)==1 && l==1 && !memcmp(buf,"a",1));
+  /* how about unicode escape */
+  assert(scan_jsonescape("a\\u005cb",buf,&l)==8 && l==3 && !memcmp(buf,"a\\b",3));
+  /* a trailing surrogate pair with no lead before it */
+  assert(scan_jsonescape("a\\udead\"",buf,&l)==1);
+  /* a lead surrogate pair with no trailer behind it */
+  assert(scan_jsonescape("a\\udafd\"",buf,&l)==1);
+  assert(scan_jsonescape("a\\udafd\\udafd",buf,&l)==1);
+  assert(scan_jsonescape("a\\udafd0",buf,&l)==1);
+  /* correct surrogate pair */
+  assert(scan_jsonescape("a\\ud834\\udd1eb",buf,&l)==14 && l==6 && !memcmp(buf,"a\xf0\x9d\x84\x9e""b",6));
+  /* how about some incorrect UTF-8? */
+  assert(scan_jsonescape("a\xc0\xaf",buf,&l)==1 && l==1 && !memcmp(buf,"a",1));
+}

textcode/scan_jsonescape.c

@@ -9,15 +9,16 @@ size_t scan_jsonescape(const char *src,char *dest,size_t *destlen) {
   unsigned int prev,cur,todo;
   prev=cur=(unsigned int)-1;
   for (i=0; s[i]; ++i) {
+    if (s[i]=='"') {
+      if (prev!=(unsigned int)-1) goto abort;
+      goto done;
+    }
     if ((c=s[i])=='\\') {
       switch (s[i+1]) {
-      case '"':
-	if (prev!=(unsigned int)-1) return 0; // lead surrogate not followed by tail surrogate
-	goto done;
       case '\\':
-	if (prev!=(unsigned int)-1) return 0; // lead surrogate not followed by tail surrogate
 	// c='\\';	// c already is backslash
 	break;
+      case '"': c='"'; break;
       case 'n': c='\n'; break;
       case 'r': c='\r'; break;
       case 'b': c='\b'; break;
@@ -28,18 +29,19 @@ size_t scan_jsonescape(const char *src,char *dest,size_t *destlen) {
 	  size_t j;
 	  for (cur=j=0; j<4; ++j) {
 	    char x=scan_fromhex(s[i+2+j]);
-	    if (x<0) return 0;	// not hex -> invalid input
+	    if (x<0) goto abort;	// not hex -> invalid input
 	    cur=(cur<<4) | x;
 	  }
 	  if (cur>=0xd800 && cur<=0xdbff) {
 	    // utf-16 surrogate pair; needs to be followed by another
 	    // surrogate. We need to read both and convert to UTF-8
-	    if (prev!=(unsigned int)-1) return 0;	// two lead surrogates
+	    if (prev!=(unsigned int)-1) goto abort;	// two lead surrogates
 	    prev=cur;
 	    i+=5;	// we want i to go up by 6, 1 is done by the for loop
 	    continue;	// write nothing!
 	  } else if (cur>=0xdc00 && cur<=0xdfff) {
-	    todo=(cur&0x3ff) | ((prev&0x3ff) << 10) | 0x100000;
+	    if (prev==(unsigned int)-1) goto abort;	// no lead surrogate
+	    todo=(cur&0x3ff) | ((prev&0x3ff) << 10) | 0x10000;
 	  } else
 	    todo=cur;
 	  written+=fmt_utf8(dest?dest+written:dest,todo);
@@ -48,16 +50,22 @@ size_t scan_jsonescape(const char *src,char *dest,size_t *destlen) {
 	  continue;
 	}
       default:
-	if (prev!=(unsigned int)-1) return 0; // lead surrogate not followed by tail surrogate
 	c=s[i+1];
 	break;
       }
       ++i;
     }
+    if (prev!=(unsigned int)-1) goto abort;
+    /* We expect utf-8 incoming. Make sure it's valid. */
+    if (!scan_utf8(s+i,4,NULL)) goto abort;
     if (dest) dest[written]=c;
     ++written;
   }
 done:
   *destlen=written;
   return i;
+abort:
+  if (prev!=(unsigned int)-1) i-=6; // if we abort and there still was an open surrogate pair, cancel it
+  *destlen=written;
+  return i;
 }