Add read_pipe_all()

Add a convenience function to read from a pipe until all requested data
has been read.

app/src/util/process.c

@@ -19,3 +19,18 @@ process_check_success(process_t proc, const char *name, bool close) {
     }
     return true;
 }
+
+ssize_t
+read_pipe_all(pipe_t pipe, char *data, size_t len) {
+    size_t copied = 0;
+    while (len > 0) {
+        ssize_t r = read_pipe(pipe, data, len);
+        if (r <= 0) {
+            return copied ? (ssize_t) copied : r;
+        }
+        len -= r;
+        data += r;
+        copied += r;
+    }
+    return copied;
+}

app/src/util/process.h

@@ -91,6 +91,9 @@ is_regular_file(const char *path);
 ssize_t
 read_pipe(pipe_t pipe, char *data, size_t len);
 
+ssize_t
+read_pipe_all(pipe_t pipe, char *data, size_t len);
+
 void
 close_pipe(pipe_t pipe);