Acquire tty if interactive when running builtins

When running a builtin, if we are an interactive shell and stdin is a tty,
then acquire ownership of the terminal via tcgetpgrp() before running the
builtin, and set it back after.

Fixes #4540

src/builtin.cpp

@@ -500,7 +500,15 @@ int builtin_run(parser_t &parser, wchar_t **argv, io_streams_t &streams) {
     }
 
     if (const builtin_data_t *data = builtin_lookup(argv[0])) {
-        return data->func(parser, streams, argv);
+        // If we are interactive, save the foreground pgroup and restore it after in case the
+        // builtin needs to read from the terminal. See #4540.
+        bool grab_tty = is_interactive_session && isatty(streams.stdin_fd);
+        pid_t pgroup_to_restore = grab_tty ? terminal_acquire_before_builtin() : -1;
+        int ret = data->func(parser, streams, argv);
+        if (pgroup_to_restore >= 0) {
+            tcsetpgrp(STDIN_FILENO, pgroup_to_restore);
+        }
+        return ret;
     }
 
     debug(0, UNKNOWN_BUILTIN_ERR_MSG, argv[0]);

src/proc.cpp

@@ -870,6 +870,17 @@ bool terminal_give_to_job(const job_t *j, bool cont) {
     return true;
 }
 
+pid_t terminal_acquire_before_builtin() {
+    pid_t selfpid = getpid();
+    pid_t current_owner = tcgetpgrp(STDIN_FILENO);
+    if (current_owner >= 0 && current_owner != selfpid) {
+        if (tcsetpgrp(STDIN_FILENO, selfpid) == 0) {
+            return current_owner;
+        }
+    }
+    return -1;
+}
+
 /// Returns control of the terminal to the shell, and saves the terminal attribute state to the job,
 /// so that we can restore the terminal ownership to the job at a later time.
 static bool terminal_return_from_job(job_t *j) {

src/proc.h

@@ -373,3 +373,7 @@ pid_t proc_wait_any();
 #endif
 
 bool terminal_give_to_job(const job_t *j, bool cont);
+
+/// Given that we are about to run a builtin, acquire the terminal if we do not own it. Returns the
+/// pid to restore after running the builtin, or -1 if there is no pid to restore.
+pid_t terminal_acquire_before_builtin();

tests/read.expect

@@ -90,6 +90,14 @@ expect_prompt
 expect_marker 7
 print_var_contents foo
 
+# Verify we don't hang on `read | cat`. See #4540.
+send_line "read | cat"
+expect_read_prompt
+send_line -h "bar\r_marker 4540"
+expect_prompt
+expect_marker 4540
+
+
 # ==========
 # The fix for issue #2007 initially introduced a problem when using a function
 # to read from /dev/stdin when that is associated with the tty. These tests