echo: Buffer output and write it in one go

`streams.out.push_back` for fd_streams_t writes to the
fd *immediately*. We might want to introduce a general buffering
strategy, but in this case writing it in one go is the simplest and
seems acceptable - we already have constrained the argument size, so
just pushing it out should work well enough.

See #7836

src/builtin_echo.cpp

@@ -207,16 +207,19 @@ maybe_t<int> builtin_echo(parser_t &parser, io_streams_t &streams, wchar_t **arg
     bool continue_output = true;
 
     const wchar_t *const *args_to_echo = argv + optind;
+    // We buffer output so we can write in one go,
+    // this matters when writing to an fd.
+    wcstring out;
     for (size_t idx = 0; continue_output && args_to_echo[idx] != nullptr; idx++) {
         if (opts.print_spaces && idx > 0) {
-            streams.out.push_back(' ');
+            out.push_back(' ');
         }
 
         const wchar_t *str = args_to_echo[idx];
         for (size_t j = 0; continue_output && str[j]; j++) {
             if (!opts.interpret_special_chars || str[j] != L'\\') {
                 // Not an escape.
-                streams.out.push_back(str[j]);
+                out.push_back(str[j]);
             } else {
                 // Most escapes consume one character in addition to the backslash; the numeric
                 // sequences may consume more, while an unrecognized escape sequence consumes none.
@@ -286,13 +289,18 @@ maybe_t<int> builtin_echo(parser_t &parser, io_streams_t &streams, wchar_t **arg
                 j += consumed;
 
                 if (continue_output) {
-                    streams.out.push_back(wc);
+                    out.push_back(wc);
                 }
             }
         }
     }
     if (opts.print_newline && continue_output) {
-        streams.out.push_back('\n');
+        out.push_back('\n');
     }
+
+    if (!out.empty()) {
+        streams.out.append(out);
+    }
+
     return STATUS_CMD_OK;
 }