readch_timed to block signals

readch_timed is called after reading the escape character \x1b. The escape
char may be a standalone key press or part of an escape sequence; fish
waits for a little bit (per the fish_escape_delay_ms variable) to see if
something else arrives, before treating it as standalone escape-key press.

It may happen that a signal is delivered while fish waits. Prior to this
change we would treat this signal as a "nothing was read" event, causing
escape to be wrongly treated as standalone.

Avoid this by using pselect() with a full signal mask, to ensure this call
completes.

src/input_common.cpp

@@ -2,6 +2,7 @@
 #include "config.h"
 
 #include <errno.h>
+#include <signal.h>
 #include <unistd.h>
 
 #include <cstring>
@@ -10,6 +11,7 @@
 #endif
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/select.h>
 #include <sys/time.h>
 #include <sys/types.h>
 
@@ -223,9 +225,27 @@ maybe_t<char_event_t> input_event_queue_t::readch_timed() {
     if (auto evt = try_pop()) {
         return evt;
     }
-    const uint64_t usec_per_msec = 1000;
-    uint64_t timeout_usec = static_cast<uint64_t>(wait_on_escape_ms) * usec_per_msec;
-    if (fd_readable_set_t::is_fd_readable(in_, timeout_usec)) {
+    // We are not prepared to handle a signal immediately; we only want to know if we get input on
+    // our fd before the timeout. Use pselect to block all signals; we will handle signals
+    // before the next call to getch().
+    sigset_t sigs;
+    sigfillset(&sigs);
+
+    // pselect expects timeouts in nanoseconds.
+    const uint64_t nsec_per_msec = 1000 * 1000;
+    const uint64_t nsec_per_sec = nsec_per_msec * 1000;
+    const uint64_t wait_nsec = wait_on_escape_ms * nsec_per_msec;
+    struct timespec timeout;
+    timeout.tv_sec = (wait_nsec) / nsec_per_sec;
+    timeout.tv_nsec = (wait_nsec) % nsec_per_sec;
+
+    // We have one fd of interest.
+    fd_set fdset;
+    FD_ZERO(&fdset);
+    FD_SET(in_, &fdset);
+
+    int res = pselect(in_ + 1, &fdset, nullptr, nullptr, &timeout, &sigs);
+    if (res > 0) {
         return readch();
     }
     return none();