common: simplify and optimize string escape parsing

This code is shared between input.conf parser and option parser. Until
now, the performance didn't really matter. But I want to use this code
for JSON parsing too, and since JSON will have to be parsed a lot, it
should probably try to avoid realloc'ing too much.

This commit moves parsing of C-style escaped strings into a common
function, and allows using it in a way realloc can be completely
avoided, if the already allocated buffer is large enough.

common/common.c

@@ -115,12 +115,22 @@ char *mp_append_utf8_buffer(char *buffer, uint32_t codepoint)
     return talloc_strndup_append_buffer(buffer, data, output - data);
 }
 
+// Like mp_append_utf8_buffer, but use bstr_xappend().
+void mp_append_utf8_bstr(void *talloc_ctx, struct bstr *buf, uint32_t codepoint)
+{
+    char data[8];
+    uint8_t tmp;
+    char *output = data;
+    PUT_UTF8(codepoint, tmp, *output++ = tmp;);
+    bstr_xappend(talloc_ctx, buf, (bstr){data, output - data});
+}
+
 // Parse a C-style escape beginning at code, and append the result to *str
 // using talloc. The input string (*code) must point to the first character
 // after the initial '\', and after parsing *code is set to the first character
 // after the current escape.
 // On error, false is returned, and all input remains unchanged.
-bool mp_parse_escape(bstr *code, char **str)
+static bool mp_parse_escape(void *talloc_ctx, bstr *dst, bstr *code)
 {
     if (code->len < 1)
         return false;
@@ -137,7 +147,7 @@ bool mp_parse_escape(bstr *code, char **str)
     case '\'': replace = '\''; break;
     }
     if (replace) {
-        *str = talloc_strndup_append_buffer(*str, &replace, 1);
+        bstr_xappend(talloc_ctx, dst, (bstr){&replace, 1});
         *code = bstr_cut(*code, 1);
         return true;
     }
@@ -146,7 +156,7 @@ bool mp_parse_escape(bstr *code, char **str)
         char c = bstrtoll(num, &num, 16);
         if (!num.len)
             return false;
-        *str = talloc_strndup_append_buffer(*str, &c, 1);
+        bstr_xappend(talloc_ctx, dst, (bstr){&c, 1});
         *code = bstr_cut(*code, 3);
         return true;
     }
@@ -155,9 +165,64 @@ bool mp_parse_escape(bstr *code, char **str)
         int c = bstrtoll(num, &num, 16);
         if (num.len)
             return false;
-        *str = mp_append_utf8_buffer(*str, c);
+        mp_append_utf8_bstr(talloc_ctx, dst, c);
         *code = bstr_cut(*code, 5);
         return true;
     }
     return false;
 }
+
+// Like mp_append_escaped_string, but set *dst to sliced *src if no escape
+// sequences have to be parsed (i.e. no memory allocation is required), and
+// if dst->start was NULL on function entry.
+bool mp_append_escaped_string_noalloc(void *talloc_ctx, bstr *dst, bstr *src)
+{
+    bstr t = *src;
+    int cur = 0;
+    while (1) {
+        if (cur >= t.len || t.start[cur] == '"') {
+            *src = bstr_cut(t, cur);
+            t = bstr_splice(t, 0, cur);
+            if (dst->start == NULL) {
+                *dst = t;
+            } else {
+                bstr_xappend(talloc_ctx, dst, t);
+            }
+            return true;
+        } else if (t.start[cur] == '\\') {
+            bstr_xappend(talloc_ctx, dst, bstr_splice(t, 0, cur));
+            t = bstr_cut(t, cur + 1);
+            cur = 0;
+            if (!mp_parse_escape(talloc_ctx, dst, &t))
+                goto error;
+        } else {
+            cur++;
+        }
+    }
+error:
+    return false;
+}
+
+// src is expected to point to a C-style string literal, *src pointing to the
+// first char after the starting '"'. It will append the contents of the literal
+// to *dst (using talloc_ctx) until the first '"' or the end of *str is found.
+// See bstr_xappend() how data is appended to *dst.
+// On success, *src will either start with '"', or be empty.
+// On error, return false, and *dst will contain the string until the first
+// error, *src is not changed.
+// Note that dst->start will be implicitly \0-terminated on successful return,
+// and if it was NULL or \0-terminated before calling the function.
+// As mentioned above, the caller is responsible for skipping the '"' chars.
+bool mp_append_escaped_string(void *talloc_ctx, bstr *dst, bstr *src)
+{
+    if (mp_append_escaped_string_noalloc(talloc_ctx, dst, src)) {
+        // Guarantee copy (or allocation).
+        if (!dst->start || dst->start == src->start) {
+            bstr res = *dst;
+            *dst = (bstr){0};
+            bstr_xappend(talloc_ctx, dst, res);
+        }
+        return true;
+    }
+    return false;
+}

common/common.h

@@ -76,6 +76,12 @@ bool mp_rect_intersection(struct mp_rect *rc, const struct mp_rect *rc2);
 char *mp_append_utf8_buffer(char *buffer, uint32_t codepoint);
 
 struct bstr;
-bool mp_parse_escape(struct bstr *code, char **str);
+
+void mp_append_utf8_bstr(void *talloc_ctx, struct bstr *buf, uint32_t codepoint);
+
+bool mp_append_escaped_string_noalloc(void *talloc_ctx, struct bstr *dst,
+                                      struct bstr *src);
+bool mp_append_escaped_string(void *talloc_ctx, struct bstr *dst,
+                              struct bstr *src);
 
 #endif /* MPLAYER_MPCOMMON_H */

input/cmd_parse.c

@@ -41,31 +41,6 @@ static bool read_token(bstr str, bstr *out_rest, bstr *out_token)
     return true;
 }
 
-static bool read_escaped_string(void *talloc_ctx, bstr *str, bstr *literal)
-{
-    bstr t = *str;
-    char *new = talloc_strdup(talloc_ctx, "");
-    while (t.len) {
-        if (t.start[0] == '"')
-            break;
-        if (t.start[0] == '\\') {
-            t = bstr_cut(t, 1);
-            if (!mp_parse_escape(&t, &new))
-                goto error;
-        } else {
-            new = talloc_strndup_append_buffer(new, t.start, 1);
-            t = bstr_cut(t, 1);
-        }
-    }
-    int len = str->len - t.len;
-    *literal = new ? bstr0(new) : bstr_splice(*str, 0, len);
-    *str = bstr_cut(*str, len);
-    return true;
-error:
-    talloc_free(new);
-    return false;
-}
-
 // Somewhat awkward; the main purpose is supporting both strings and
 // pre-split string arrays as input.
 struct parse_ctx {
@@ -92,7 +67,7 @@ static int pctx_read_token(struct parse_ctx *ctx, bstr *out)
         ctx->str = bstr_lstrip(ctx->str);
         bstr start = ctx->str;
         if (bstr_eatstart0(&ctx->str, "\"")) {
-            if (!read_escaped_string(ctx->tmp, &ctx->str, out)) {
+            if (!mp_append_escaped_string_noalloc(ctx->tmp, out, &ctx->str)) {
                 MP_ERR(ctx, "Broken string escapes: ...>%.*s<.\n", BSTR_P(start));
                 return -1;
             }

options/m_option.c

@@ -746,20 +746,17 @@ const m_option_type_t m_option_type_float = {
 
 static char *unescape_string(void *talloc_ctx, bstr str)
 {
-    char *res = talloc_strdup(talloc_ctx, "");
+    bstr dst = {0};
     while (str.len) {
-        bstr rest;
-        bool esc = bstr_split_tok(str, "\\", &str, &rest);
-        res = talloc_strndup_append_buffer(res, str.start, str.len);
-        if (esc) {
-            if (!mp_parse_escape(&rest, &res)) {
-                talloc_free(res);
-                return NULL;
-            }
+        if (!mp_append_escaped_string(talloc_ctx, &dst, &str)) {
+            talloc_free(dst.start);
+            return NULL;
         }
-        str = rest;
+        if (!bstr_eatstart0(&str, "\""))
+            break;
+        bstr_xappend(talloc_ctx, &dst, bstr0("\""));
     }
-    return res;
+    return dst.start;
 }
 
 static char *escape_string(char *str0)