content_encoding: make Curl_all_content_encodings allocless

- Fixes a memory leak pointed out by Coverity
- Also found by OSS-Fuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=63947
- Avoids unncessary allocations

Follow-up ad051e1cbe

Closes #12289

lib/content_encoding.c

@@ -849,13 +849,17 @@ static const struct Curl_cwtype * const encodings[] = {
 };
 
 
-/* Return a list of comma-separated names of supported encodings. */
-char *Curl_all_content_encodings(void)
+/* Provide a list of comma-separated names of supported encodings.
+*/
+void Curl_all_content_encodings(char *buf, size_t blen)
 {
   size_t len = 0;
   const struct Curl_cwtype * const *cep;
   const struct Curl_cwtype *ce;
-  char *ace;
+
+  DEBUGASSERT(buf);
+  DEBUGASSERT(blen);
+  buf[0] = 0;
 
   for(cep = encodings; *cep; cep++) {
     ce = *cep;
@@ -863,12 +867,12 @@ char *Curl_all_content_encodings(void)
       len += strlen(ce->name) + 2;
   }
 
-  if(!len)
-    return strdup(CONTENT_ENCODING_DEFAULT);
-
-  ace = malloc(len);
-  if(ace) {
-    char *p = ace;
+  if(!len) {
+    if(blen >= sizeof(CONTENT_ENCODING_DEFAULT))
+      strcpy(buf, CONTENT_ENCODING_DEFAULT);
+  }
+  else if(blen > len) {
+    char *p = buf;
     for(cep = encodings; *cep; cep++) {
       ce = *cep;
       if(!strcasecompare(ce->name, CONTENT_ENCODING_DEFAULT)) {
@@ -880,11 +884,8 @@ char *Curl_all_content_encodings(void)
     }
     p[-2] = '\0';
   }
-
-  return ace;
 }
 
-
 /* Deferred error dummy writer. */
 static CURLcode error_do_init(struct Curl_easy *data,
                                   struct Curl_cwriter *writer)
@@ -898,7 +899,8 @@ static CURLcode error_do_write(struct Curl_easy *data,
                                      struct Curl_cwriter *writer, int type,
                                      const char *buf, size_t nbytes)
 {
-  char *all = Curl_all_content_encodings();
+  char all[256];
+  (void)Curl_all_content_encodings(all, sizeof(all));
 
   (void) writer;
   (void) buf;
@@ -907,11 +909,8 @@ static CURLcode error_do_write(struct Curl_easy *data,
   if(!(type & CLIENTWRITE_BODY))
     return Curl_cwriter_write(data, writer->next, type, buf, nbytes);
 
-  if(!all)
-    return CURLE_OUT_OF_MEMORY;
   failf(data, "Unrecognized content encoding type. "
         "libcurl understands %s content encodings.", all);
-  free(all);
   return CURLE_BAD_CONTENT_ENCODING;
 }
 
@@ -1021,9 +1020,15 @@ CURLcode Curl_build_unencoding_stack(struct Curl_easy *data,
   return CURLE_NOT_BUILT_IN;
 }
 
-char *Curl_all_content_encodings(void)
+void Curl_all_content_encodings(char *buf, size_t blen)
 {
-  return strdup(CONTENT_ENCODING_DEFAULT);  /* Satisfy caller. */
+  DEBUGASSERT(buf);
+  DEBUGASSERT(blen);
+  if(blen < sizeof(CONTENT_ENCODING_DEFAULT))
+    buf[0] = 0;
+  else
+    strcpy(buf, CONTENT_ENCODING_DEFAULT);
 }
 
+
 #endif /* CURL_DISABLE_HTTP */

lib/content_encoding.h

@@ -27,7 +27,7 @@
 
 struct Curl_cwriter;
 
-char *Curl_all_content_encodings(void);
+void Curl_all_content_encodings(char *buf, size_t blen);
 
 CURLcode Curl_build_unencoding_stack(struct Curl_easy *data,
                                      const char *enclist, int is_transfer);

lib/setopt.c

@@ -592,13 +592,9 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
      */
     argptr = va_arg(param, char *);
     if(argptr && !*argptr) {
-      argptr = Curl_all_content_encodings();
-      if(!argptr)
-        result = CURLE_OUT_OF_MEMORY;
-      else {
-        result = Curl_setstropt(&data->set.str[STRING_ENCODING], argptr);
-        free(argptr);
-      }
+      char all[256];
+      Curl_all_content_encodings(all, sizeof(all));
+      result = Curl_setstropt(&data->set.str[STRING_ENCODING], all);
     }
     else
       result = Curl_setstropt(&data->set.str[STRING_ENCODING], argptr);