Clean up and relnote shebangless script support

This adds a test for shebangless support from #7802, cleans up some of
its tricks, and includes it in the changelog.

CHANGELOG.rst

@@ -13,6 +13,7 @@ Deprecations and removed features
 Scripting improvements
 ----------------------
 - ``math`` now has a ``log2`` function like the documentation already claimed.
+- Shebang lines are no longer required within shell scripts, improving support for scripts with concatenated binary contents. If a file fails to execute and passes a binary safety check, fish will re-invoke the file using `/bin/sh` (:issue:`7802`).
 
 Interactive improvements
 -------------------------

src/exec.cpp

@@ -63,6 +63,9 @@ enum class launch_result_t {
     failed,
 } __warn_unused_type;
 
+/// This is a 'looks like text' check.
+/// \return true if either there is no NUL byte, or there is a line containing a lowercase letter
+/// before the first NUL byte.
 static bool is_thompson_shell_payload(const char *p, size_t n) {
     if (!memchr(p, '\0', n)) return true;
     bool haslower = false;
@@ -87,15 +90,16 @@ static bool is_thompson_shell_payload(const char *p, size_t n) {
 /// is usually uppercase, e.g. PNG, JFIF, MZ, etc. These rules are also
 /// flexible enough to permit scripts with concatenated binary content,
 /// such as Actually Portable Executable.
+/// N.B.: this is called after fork, it must not allocate heap memory.
 bool is_thompson_shell_script(const char *path) {
     int e = errno;
     bool res = false;
-    int fd = open(path, O_RDONLY | O_NOCTTY);
+    int fd = open_cloexec(path, O_RDONLY | O_NOCTTY);
     if (fd != -1) {
         char buf[256];
         ssize_t got = read(fd, buf, sizeof(buf));
         close(fd);
-        if (got != -1 && is_thompson_shell_payload(buf, got)) {
+        if (got >= 0 && is_thompson_shell_payload(buf, static_cast<size_t>(got))) {
             res = true;
         }
     }
@@ -119,12 +123,22 @@ bool is_thompson_shell_script(const char *path) {
     err = errno;
 
     // The shebang wasn't introduced until UNIX Seventh Edition, so if
-    // the kernel won't run the binary we hand it off to the intpreter
+    // the kernel won't run the binary we hand it off to the interpreter
     // after performing a binary safety check, recommended by POSIX: a
     // line needs to exist before the first \0 with a lowercase letter
     if (err == ENOEXEC && is_thompson_shell_script(actual_cmd)) {
-        *--argv = const_cast<char *>(_PATH_BSHELL);
-        execve(_PATH_BSHELL, argv, envv);
+        // Construct new argv.
+        // We must not allocate memory, so only 128 args are supported.
+        constexpr size_t maxargs = 128;
+        size_t nargs = 0;
+        while (argv[nargs]) nargs++;
+        if (nargs <= maxargs) {
+            char *argv2[1 + maxargs + 1];  // +1 for /bin/sh, +1 for terminating nullptr
+            char interp[] = _PATH_BSHELL;
+            argv2[0] = interp;
+            std::copy_n(argv, 1 + nargs, &argv2[1]);  // +1 to copy terminating nullptr
+            execve(_PATH_BSHELL, argv2, envv);
+        }
     }
 
     errno = err;

src/postfork.cpp

@@ -309,15 +309,15 @@ maybe_t<pid_t> posix_spawner_t::spawn(const char *cmd, char *const argv[], char
         // line needs to exist before the first \0 with a lowercase letter
         if (error_ == ENOEXEC && is_thompson_shell_script(cmd)) {
             error_ = 0;
-            size_t n = 0;
-            while (argv[n]) ++n;
-            std::unique_ptr<char *[]> argv2(new char *[1 + n + 1]);
+            // Create a new argv with /bin/sh prepended.
+            std::vector<char *> argv2;
             char interp[] = _PATH_BSHELL;
-            argv2[0] = interp;
-            for (size_t i = 0; i < n + 1; ++i) {
-                argv2[i + 1] = argv[i];
+            argv2.push_back(interp);
+            for (size_t i = 0; argv[i] != nullptr; i++) {
+                argv2.push_back(argv[i]);
             }
-            if (check_fail(posix_spawn(&pid, interp, &*actions_, &*attr_, argv2.get(), envp))) {
+            argv2.push_back(nullptr);
+            if (check_fail(posix_spawn(&pid, interp, &*actions_, &*attr_, &argv2[0], envp))) {
                 return none();
             }
         } else {

tests/checks/noshebang.fish

@@ -0,0 +1,66 @@
+# RUN: %fish %s
+
+# Test for shebangless scripts - see 7802.
+
+set testdir (mktemp -d)
+cd $testdir
+
+touch file
+chmod a+x file
+
+function runfile
+    # Run our file twice, printing status.
+    # Arguments are passed to exercise the re-execve code paths; they have no other effect.
+    set -g fish_use_posix_spawn 0
+    ./file arg1 arg2 arg3
+    echo $status
+
+    set -g fish_use_posix_spawn 1
+    ./file arg1 arg2 arg3 arg4 arg5
+    echo $status
+end
+
+# Empty executable files are 'true'.
+true >file
+runfile
+#CHECK: 0
+#CHECK: 0
+
+# Files without NUL are 'true' as well.
+echo -e -n '#COMMENT\n#COMMENT' >file
+runfile
+#CHECK: 0
+#CHECK: 0
+
+# On to NUL bytes.
+# The heuristic is that there must be a line containing a lowercase letter before the first NUL byte.
+echo -n -e 'true\n\x00' >file
+runfile
+#CHECK: 0
+#CHECK: 0
+
+# Doesn't meet our heuristic as there is no newline.
+echo -n -e 'true\x00' >file
+runfile
+#CHECK: 125
+#CHECKERR: Failed {{.*}}
+#CHECKERR: exec: {{.*}}
+#CHECKERR: {{.*}}
+
+#CHECK: 125
+#CHECKERR: Failed {{.*}}
+#CHECKERR: exec: {{.*}}
+#CHECKERR: {{.*}}
+
+# Doesn't meet our heuristic as there is no lowercase before newline.
+echo -n -e 'NOPE\n\x00' >file
+runfile
+#CHECK: 125
+#CHECKERR: Failed {{.*}}
+#CHECKERR: exec: {{.*}}
+#CHECKERR: {{.*}}
+
+#CHECK: 125
+#CHECKERR: Failed {{.*}}
+#CHECKERR: exec: {{.*}}
+#CHECKERR: {{.*}}