Continue reading (and discarding) output if log buffer full

Rather than ending the consumption of output, continue to consume it.
This means output may be lost, but avoids the possibility of blocking a
service execution because its output buffer becomes full.

src/proc-service.cc

@@ -261,42 +261,57 @@ rearm log_output_watcher::fd_event(eventloop_t &eloop, int fd, int flags) noexce
 {
 	// In case buffer size has been decreased, check if we are already at the limit:
 	if (service->log_buf_size >= service->log_buf_max) {
-		return rearm::DISARM;
+		// If so, read and discard.
+		char buf[1024];
+		int r = bp_sys::read(fd, buf, 1024);
+		if (r == -1) {
+	    	if (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR) {
+	    		return rearm::REARM;
+	    	}
+	    	goto bad_read;
+		}
+		if (r == 0) goto eof_read;
+		return rearm::REARM;
 	}
 
-    size_t max_read = std::min(service->log_buf_max / 8, service->log_buf_max - service->log_buf_size);
+	{
+		size_t max_read = std::max(service->log_buf_max / 8, 256u);
+		max_read = std::min((unsigned)max_read, service->log_buf_max - service->log_buf_size);
 
-    // ensure vector has size sufficient to read
-    unsigned new_size = service->log_buf_size + max_read;
-    if (!service->ensure_log_buffer_backing(new_size)) {
-    	return rearm::DISARM;
-    }
+		// ensure vector has size sufficient to read
+		unsigned new_size = service->log_buf_size + max_read;
+		if (!service->ensure_log_buffer_backing(new_size)) {
+			return rearm::DISARM;
+		}
 
-    max_read = service->log_buffer.size() - service->log_buf_size;
+		max_read = service->log_buffer.size() - service->log_buf_size;
 
-    int r = bp_sys::read(fd, service->log_buffer.data() + service->log_buf_size, max_read);
-    if (r == -1) {
-    	if (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR) {
-    		return rearm::REARM;
-    	}
-    	log(loglevel_t::WARN, "Service ", service->get_name(), " output not readable: ", strerror(errno));
-    }
+		int r = bp_sys::read(fd, service->log_buffer.data() + service->log_buf_size, max_read);
+		if (r == -1) {
+			if (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR) {
+				return rearm::REARM;
+			}
+			goto bad_read;
+		}
 
-    if (r <= 0) {
-    	deregister(eloop);
-        close(fd);
-        close(service->log_output_fd);
-        service->log_input_fd = -1;
-        service->log_output_fd = -1;
-        return rearm::REMOVED;
-    }
+		if (r == 0) goto eof_read;
 
-    service->log_buf_size += r;
-    if (service->log_buf_size >= service->log_buf_max) {
-    	return rearm::DISARM;
-    }
+		service->log_buf_size += r;
+		return rearm::REARM;
+	}
 
-    return rearm::REARM;
+    // error/end-of-stream handling:
+
+    bad_read:
+	log(loglevel_t::WARN, "Service ", service->get_name(), " output not readable: ", strerror(errno));
+
+	eof_read:
+	deregister(eloop);
+    close(fd);
+    close(service->log_output_fd);
+    service->log_input_fd = -1;
+    service->log_output_fd = -1;
+    return rearm::REMOVED;
 }
 
 void process_service::handle_exit_status(bp_sys::exit_status exit_status) noexcept