log_proxy_stderr: limit the length of Event Log lines.

If a proxy command jabbers on standard error in a way that doesn't
involve any newline characters, we now won't keep buffering data for
ever.

(Not that I've heard of it happening, but I noticed the theoretical
possibility on the way past in a recent cleanup pass.)

be_misc.c

@@ -59,12 +59,15 @@ void backend_socket_log(Seat *seat, LogContext *logctx,
     }
 }
 
-void log_proxy_stderr(Plug *plug, bufchain *buf, const void *vdata, size_t len)
+void psb_init(ProxyStderrBuf *psb)
+{
+    psb->size = 0;
+}
+
+void log_proxy_stderr(Plug *plug, ProxyStderrBuf *psb,
+                      const void *vdata, size_t len)
 {
     const char *data = (const char *)vdata;
-    size_t pos = 0;
-    const char *nlpos;
-    char *msg, *fullmsg;
 
     /*
      * This helper function allows us to collect the data written to a
@@ -72,43 +75,77 @@ void log_proxy_stderr(Plug *plug, bufchain *buf, const void *vdata, size_t len)
      * happen to get from its pipe, and whenever we have a complete
      * line, we pass it to plug_log.
      *
-     * Prerequisites: a plug to log to, and a bufchain stored
-     * somewhere to collect the data in.
+     * (We also do this when the buffer in psb fills up, to avoid just
+     * allocating more and more memory forever, and also to keep Event
+     * Log lines reasonably bounded in size.)
+     *
+     * Prerequisites: a plug to log to, and a ProxyStderrBuf stored
+     * somewhere to collect any not-yet-output partial line.
      */
 
-    while (pos < len && (nlpos = memchr(data+pos, '\n', len-pos)) != NULL) {
+    while (len > 0) {
         /*
-         * Found a newline in the current input buffer. Append it to
-         * the bufchain (which may contain a partial line from last
-         * time).
+         * Copy as much data into psb->buf as will fit.
          */
-        bufchain_add(buf, data + pos, nlpos - (data + pos));
+        assert(psb->size < lenof(psb->buf));
+        size_t to_consume = lenof(psb->buf) - psb->size;
+        if (to_consume > len)
+            to_consume = len;
+        memcpy(psb->buf + psb->size, data, to_consume);
+        data += to_consume;
+        len -= to_consume;
+        psb->size += to_consume;
 
         /*
-         * Collect the resulting line of data and pass it to plug_log.
+         * Output any full lines in psb->buf.
          */
-        size_t msglen = bufchain_size(buf);
-        assert(msglen < ~(size_t)0);
-        msg = snewn(msglen+1, char);
-        bufchain_fetch(buf, msg, msglen);
-        bufchain_consume(buf, msglen);
-        while (msglen > 0 && (msg[msglen-1] == '\n' || msg[msglen-1] == '\r'))
-            msglen--;
-        msg[msglen] = '\0';
-        fullmsg = dupprintf("proxy: %s", msg);
-        plug_log(plug, 2, NULL, 0, fullmsg, 0);
-        sfree(fullmsg);
-        sfree(msg);
+        size_t pos = 0;
+        while (pos < psb->size) {
+            char *nlpos = memchr(psb->buf + pos, '\n', psb->size - pos);
+            if (!nlpos)
+                break;
+
+            /*
+             * Found a newline in the buffer, so we can output a line.
+             */
+            size_t endpos = nlpos - psb->buf;
+            while (endpos > pos && (psb->buf[endpos-1] == '\n' ||
+                                    psb->buf[endpos-1] == '\r'))
+                endpos--;
+            char *msg = dupprintf(
+                "proxy: %.*s", (int)(endpos - pos), psb->buf + pos);
+            plug_log(plug, 2, NULL, 0, msg, 0);
+            sfree(msg);
+
+            pos = nlpos - psb->buf + 1;
+            assert(pos <= psb->size);
+        }
 
         /*
-         * Advance past the newline.
+         * If the buffer is completely full and we didn't output
+         * anything, then output the whole thing, flagging it as a
+         * truncated line.
+         */
+        if (pos == 0 && psb->size == lenof(psb->buf)) {
+            char *msg = dupprintf(
+                "proxy (partial line): %.*s", (int)psb->size, psb->buf);
+            plug_log(plug, 2, NULL, 0, msg, 0);
+            sfree(msg);
+
+            pos = psb->size = 0;
+        }
+
+        /*
+         * Now move any remaining data up to the front of the buffer.
+         */
+        size_t newsize = psb->size - pos;
+        if (newsize)
+            memmove(psb->buf, psb->buf + pos, newsize);
+        psb->size = newsize;
+
+        /*
+         * And loop round again if there's more data to be read from
+         * our input.
          */
-        pos += nlpos+1 - (data + pos);
     }
-
-    /*
-     * Now any remaining data is a partial line, which we save for
-     * next time.
-     */
-    bufchain_add(buf, data + pos, len - pos);
 }