input: arguments quoting: support single-quotes

Users expect single quotes to work when the value includes literal
backslashes or double-quotes (or as general quoting like in shell).

The updated docs also include some previously-missing notes:
- newline is only supported in double quotes.
- adjacent (quoted) arguments don't join into one.

Supporting mixed quoting (adjacent quoted strings) would make
mpv's parsing more complete, but would require delicate effort of
larger scope, for two reasons:
- We'd need to also support escaping outside of quotes and do our
  best about backward compatibility.
- The parsed value can either be a substring of the input or
  a newly-allocated string, which would be delicate when joining.

Not critical to add right now.

1 files changed, 11 insertions, 1 deletions

diff --git a/input/cmd.c b/input/cmd.c
index 9f0c758b0c..0ab06e3574 100644
--- a/input/cmd.c
+++ b/input/cmd.c
@@ -336,11 +336,21 @@ static int pctx_read_token(struct parse_ctx *ctx, bstr *out)
             return -1;
         }
         if (!bstr_eatstart0(&ctx->str, "\"")) {
-            MP_ERR(ctx, "Unterminated quotes: ...>%.*s<.\n", BSTR_P(start));
+            MP_ERR(ctx, "Unterminated double quote: ...>%.*s<.\n", BSTR_P(start));
             return -1;
         }
         return 1;
     }
+    if (bstr_eatstart0(&ctx->str, "'")) {
+        int next = bstrchr(ctx->str, '\'');
+        if (next < 0) {
+            MP_ERR(ctx, "Unterminated single quote: ...>%.*s<.\n", BSTR_P(start));
+            return -1;
+        }
+        *out = bstr_splice(ctx->str, 0, next);
+        ctx->str = bstr_cut(ctx->str, next+1);
+        return 1;
+    }
     if (ctx->start.len > 1 && bstr_eatstart0(&ctx->str, "`")) {
         char endquote[2] = {ctx->str.start[0], '`'};
         ctx->str = bstr_cut(ctx->str, 1);