libsubprocess: add extra documentation

src/common/libsubprocess/fbuf.h

@@ -22,7 +22,7 @@ void fbuf_destroy (void *fb);
 /* Returns the buffer size, set when fbuf_create () was called */
 int fbuf_size (struct fbuf *fb);
 
-/* Returns the number of bytes current stored in fbuf */
+/* Returns the number of bytes currently stored in fbuf */
 int fbuf_bytes (struct fbuf *fb);
 
 /* Returns the number of bytes of space available in fbuf */

src/common/libsubprocess/fbuf_watcher.h

@@ -18,8 +18,15 @@ enum {
     FBUF_WATCHER_LINE_BUFFER = 1, /* line buffer data before invoking callback */
 };
 
-/* on eof, callback will be called with an empty buffer */
-/* if line buffered, second to last callback may not contain a full line */
+/* read watcher
+ *
+ * - data from fd copied into buffer
+ * - when data is available, triggers callback
+ * - on eof, callback will be called with an empty buffer
+ * - if line buffered, second to last callback may not contain a full line
+ * - users should read from the buffer or stop the watcher, to avoid
+ *   excessive event loop iterations without progress
+ */
 flux_watcher_t *fbuf_read_watcher_create (flux_reactor_t *r,
                                           int fd,
                                           int size,
@@ -40,7 +47,11 @@ const char *fbuf_read_watcher_get_data (flux_watcher_t *w, int *lenp);
 void fbuf_read_watcher_incref (flux_watcher_t *w);
 void fbuf_read_watcher_decref (flux_watcher_t *w);
 
-/* 'cb' only called after fd closed (FLUX_POLLOUT) or error (FLUX_POLLERR) */
+/* write watcher
+ *
+ * - data from buffer written to fd
+ * - callback triggered after fd closed (FLUX_POLLOUT) or error (FLUX_POLLERR)
+ */
 flux_watcher_t *fbuf_write_watcher_create (flux_reactor_t *r,
                                            int fd,
                                            int size,

src/common/libsubprocess/local.c

@@ -154,14 +154,14 @@ static void local_out_cb (flux_reactor_t *r,
 }
 
 static void local_stdout_cb (flux_reactor_t *r, flux_watcher_t *w,
-                               int revents, void *arg)
+                             int revents, void *arg)
 {
     struct subprocess_channel *c = (struct subprocess_channel *)arg;
     local_output (c, w, revents, c->p->ops.on_stdout);
 }
 
 static void local_stderr_cb (flux_reactor_t *r, flux_watcher_t *w,
-                               int revents, void *arg)
+                             int revents, void *arg)
 {
     struct subprocess_channel *c = (struct subprocess_channel *)arg;
     local_output (c, w, revents, c->p->ops.on_stderr);

src/common/libsubprocess/subprocess.h

@@ -88,6 +88,11 @@ typedef void (*flux_subprocess_hook_f) (flux_subprocess_t *p, void *arg);
 /*
  *  Functions for event-driven subprocess handling:
  *
+ *  When output callbacks are called, flux_subprocess_read(),
+ *  flux_subprocess_read_line() and similar functions should be used
+ *  to read buffered data.  If this is not done, it can lead to
+ *  excessive callbacks and code "spinning".
+ *
  */
 typedef struct {
     flux_subprocess_f on_completion;    /* Process exited and all I/O