[core] Implement POSIX rule for $PWD

This fixes the Perl build failure.

If $PWD is an absolute path, and it's the same file as '.', then use it
as our "cached" $PWD value.

This means that symlinks to $PWD work.

Otherwise, recompute it.

Thanks to Samuel for finding this bug, and narrowing it down.

It still took some effort to find on top:

- grepping through dash source code (but missing this important part)
- uploading dash source to Claude and asking questions
- finding the reference to the POSIX rule in bash source code

Author: Andy C

core/pyos.py

@@ -294,3 +294,24 @@ def MakeDirCacheKey(path):
     directory accesses."""
     st = posix.stat(path)
     return (path, int(st.st_mtime))
+
+
+def IsSameFile(path1, path2):
+    # type: (str, str) -> bool
+
+    try:
+        st1 = posix.stat(path1)
+    except OSError:
+        return False
+
+    try:
+        st2 = posix.stat(path2)
+    except OSError:
+        return False
+
+    if st1.st_dev != st2.st_dev:
+        return False
+    if st1.st_ino != st2.st_ino:
+        return False
+
+    return True

core/sh_init.py

@@ -248,22 +248,28 @@ def InitVarsAfterEnv(mem, mutable_opts):
 
         # NOTE: bash also has BASHOPTS
 
+        our_pwd = None  # type: Optional[str]
         val = mem.GetValue('PWD')
-        if val.tag() == value_e.Undef:
-            state.SetGlobalString(mem, 'PWD', GetWorkingDir())
+        if val.tag() == value_e.Str:
+            env_pwd = cast(value.Str, val).s
+            # POSIX rule: PWD is inherited if it's an absolute path that corresponds to '.'
+            if env_pwd.startswith('/') and pyos.IsSameFile(env_pwd, '.'):
+                our_pwd = env_pwd
+
+        # POSIX: Otherwise, recalculate it
+        if our_pwd is None:
+            our_pwd = GetWorkingDir()
+
         # It's EXPORTED, even if it's not set.  bash and dash both do this:
         #     env -i -- dash -c env
         mem.SetNamed(location.LName('PWD'),
-                     None,
+                     value.Str(our_pwd),
                      scope_e.GlobalOnly,
                      flags=state.SetExport)
 
         # Set a MUTABLE GLOBAL that's SEPARATE from $PWD.  It's used by the 'pwd'
         # builtin, and it can't be modified by users.
-        val = mem.GetValue('PWD')
-        assert val.tag() == value_e.Str, val
-        pwd = cast(value.Str, val).s
-        mem.SetPwd(pwd)
+        mem.SetPwd(our_pwd)
 
 
 def InitInteractive(mem, sh_files, lang):

cpp/core.cc

@@ -274,7 +274,6 @@ List<int>* WaitForReading(List<int>* fd_list) {
   return ret;
 }
 
-
 IOError_OSError* FlushStdout() {
   // Flush libc buffers
   if (::fflush(stdout) != 0) {
@@ -292,6 +291,23 @@ Tuple2<BigStr*, int>* MakeDirCacheKey(BigStr* path) {
   return Alloc<Tuple2<BigStr*, int>>(path, st.st_mtime);
 }
 
+bool IsSameFile(BigStr* path1, BigStr* path2) {
+  struct stat st1, st2;
+  if (::stat(path1->data(), &st1)) {
+    return false;
+  }
+  if (::stat(path2->data(), &st2)) {
+    return false;
+  }
+  if (st1.st_dev != st2.st_dev) {
+    return false;
+  }
+  if (st1.st_ino != st2.st_ino) {
+    return false;
+  }
+  return true;
+}
+
 Tuple2<int, void*> PushTermAttrs(int fd, int mask) {
   struct termios* term_attrs =
       static_cast<struct termios*>(malloc(sizeof(struct termios)));

cpp/core.h

@@ -89,6 +89,8 @@ void PopTermAttrs(int fd, int orig_local_modes, void* term_attrs);
 
 Tuple2<BigStr*, int>* MakeDirCacheKey(BigStr* path);
 
+bool IsSameFile(BigStr* path1, BigStr* path2);
+
 }  // namespace pyos
 
 namespace pyutil {

spec/builtin-cd.test.sh

@@ -1,6 +1,6 @@
 ## compare_shells: dash bash mksh zsh
 ## oils_failures_allowed: 3
-## oils_cpp_failures_allowed: 5
+## oils_cpp_failures_allowed: 3
 
 #### cd and $PWD
 cd /
@@ -453,7 +453,11 @@ PWD = /tmp/osh-spec-cd/cpan/Encode/Byte
 /tmp/osh-spec-cd/cpan/Encode/Byte
 ## END
 
-#### getcwd() syscall is not made
+#### Survey of getcwd() syscall
+
+# This is not that important -- see core/sh_init.py
+# Instead of verifying that stat('.') == stat(PWD), which is two sycalls,
+# OSH just calls getcwd() unconditionally.
 
 # so C++ leak sanitizer  doesn't print to stderr
 export ASAN_OPTIONS='detect_leaks=0'