sh_diropenat(): General improvements (#859)

- Avoid duplicating the file descriptor from openat when
  it's already >9 to increase performance and reliability.
- Add a feature test which tests the errno resulting from
  using openat on a non-directory with O_DIRECTORY|O_SEARCH.
  On illumos (and presumably Solaris) openat can misleadingly
  fail with EACCES rather than ENOTDIR, so account for that
  by testing the given directory in question with stat(2).
- Add a feature test to see if file descriptors obtained via
  O_SEARCH or O_PATH can actually be used with fchdir(2).
  This is also relevant for open(2) because of O_SEARCH's usage
  in the fallback code in sh_subshell().
- Ensure the close-on-exec bit is set for the file descriptor
  returned by sh_diropenat().

These changes allow for the elimination of the double openat and
helps mitigate the performance loss for cd's performance as
observed after the merge of 3d7b7e1:
vide #816 (comment)

Results obtained on Linux amd64 using CCFLAGS=-Os.
Results for: 'typeset -i i; time for((i=0;i<500000;i++));do cd /dev; cd /;done'
ksh-no-openat-9b0b2603:
real    0m01.72s
ksh-dev-de530132:
real    0m03.55s
ksh-this-commit:
real    0m02.91s
ksh-this-commit-no-subshell-inode-test: (same due to no subshells)
real    0m02.90s
ksh93u+:
real    0m02.02s

I'll admit cd still isn't as fast as before, yet the trade off is
still worthwhile because ksh93 scripts are bound to use subshells
(particulary command substitutions) far more often than cd. In the
contrived benchmark cd is called 1,000,000 times, which hardly
reflects real world scripts. This also doesn't account for the
practice of calling cd from within a subshell, which is common.

Results for: 'typeset -i i; time for((i=0;i<500000;i++));do (cd /dev); (cd /);done'
ksh-no-openat-9b0b2603:
real    0m06.73s
ksh-dev-de530132:
real    0m07.18s
ksh-this-commit:
real    0m06.34s
ksh-this-commit-no-subshell-inode-test:
real    0m04.77s
ksh93u+:
real    0m05.19s

Here a small uplift can be seen relative to both of the previous
93u+m iterations, but because of the inode sanity check present
since 5ee290c cd is still a bit slower in subshells relative
to ksh93u+. (Perhaps in a future commit a faster alternative
could be looked into.)

Meanwhile the relevant shbench results are about the same (-l 10):
-----------------------------------------------------------------------------------------------------------------------------------
name           ksh-no-openat-9b0b2603  ksh-dev-de530132     ksh-this-commit      ksh-this-commit-no-inode-test  ksh93u+
-----------------------------------------------------------------------------------------------------------------------------------
fibonacci.ksh  0.247 [0.240-0.258]     0.227 [0.225-0.233]  0.235 [0.227-0.241]  0.235 [0.232-0.240]            0.258 [0.251-0.264]
parens.ksh     0.191 [0.185-0.197]     0.097 [0.095-0.098]  0.097 [0.095-0.100]  0.099 [0.095-0.101]            0.227 [0.221-0.234]
-----------------------------------------------------------------------------------------------------------------------------------

Author: JohnoKing

src/cmd/ksh93/Mamfile

@@ -83,7 +83,7 @@ make install virtual
 		exec - %{iffe_run} %{<}
 	done
 
-	loop F options externs locale cmds poll rlimits
+	loop F options externs fchdir locale cmds poll rlimits
 		make FEATURE/%{F}
 			makp features/%{F}
 			exec - %{iffe_run} %{<}
@@ -268,6 +268,7 @@ make install virtual
 	make include/io.h
 		prev %{INCLUDE_AST}/sfio.h
 		prev %{INCLUDE_AST}/ast.h
+		prev FEATURE/fchdir
 	done
 
 	make include/jobs.h
@@ -343,6 +344,7 @@ make install virtual
 			prev include/test.h
 			prev %{INCLUDE_AST}/ls.h
 			prev include/builtins.h
+			prev include/io.h
 			prev include/name.h
 			prev include/path.h
 			prev include/variables.h

src/cmd/ksh93/bltins/cd_pwd.c

@@ -33,6 +33,7 @@
 #include	"path.h"
 #include	"name.h"
 #include	"builtins.h"
+#include	"io.h"
 #include	<ls.h>
 #include	"test.h"
 
@@ -48,20 +49,40 @@ static void rehash(Namval_t *np,void *data)
 
 #if _lib_openat
 /*
- * Obtain a file handle to the directory "path" relative to directory "dir"
+ * Obtain a file descriptor >9 to the directory "path" relative to directory "dir"
  */
 int sh_diropenat(int dir, const char *path)
 {
-	int fd,shfd;
-	if((fd = openat(dir, path, O_DIRECTORY|O_NONBLOCK|O_cloexec)) < 0)
-#if O_SEARCH
-		if(errno != EACCES || (fd = openat(dir, path, O_SEARCH|O_DIRECTORY|O_NONBLOCK|O_cloexec)) < 0)
+	int fd, needs_cloexec = 0;
+	if((fd = openat(dir, path, O_DIRECTORY|O_NONBLOCK|O_cloexec|O_SEARCH)) < 0)
+	{
+#if !_openat_enotdir
+		struct stat fs;
+		int saverrno = errno;
+		/* Some OSes (e.g. illumos) misleadingly fail with EACCES rather than ENOTDIR */
+		if(errno==EACCES && !stat(path, &fs) && !S_ISDIR(fs.st_mode))
+			errno = ENOTDIR;
+		else
+			errno = saverrno;
 #endif
-			return fd;
-	/* Move fd to a number > 10 and register the fd number with the shell */
-	shfd = sh_fcntl(fd, F_dupfd_cloexec, 10);
-	close(fd);
-	return shfd;
+		return fd;
+	}
+	if(fd < 10)
+	{
+		/* Duplicate the fd and register it with the shell */
+		int shfd = sh_fcntl(fd, F_dupfd_cloexec, 10);
+		close(fd);
+		if(shfd < 0)
+			return shfd;
+		if(F_dupfd_cloexec == F_DUPFD)
+			needs_cloexec = 1;
+		fd = shfd;
+	}
+	else if(O_cloexec == 0)
+		needs_cloexec = 1;
+	if(needs_cloexec)
+		sh_fcntl(fd,F_SETFD,FD_CLOEXEC);
+	return fd;
 }
 #endif /* _lib_openat */
 

src/cmd/ksh93/features/fchdir

@@ -0,0 +1,61 @@
+tst	fchdir_osearch_compat note{ can fchdir(2) use file descriptors obtained using O_PATH or O_SEARCH }end output{
+	#include <ast.h>
+	#include <wait.h>
+	#include <stdio.h>
+	#if !O_SEARCH
+	#error No O_SEARCH or O_PATH detected; fchdir is unreliable
+	#endif
+	int main(void)
+	{
+		char	dirname[64];
+		int	n;
+		pid_t	child;
+		sprintf(dirname,".dir2.%u",(unsigned int)getpid());
+		mkdir(dirname,0777);
+		child = fork();
+		if(child==0)
+		{
+	#if _lib_openat
+			if((n = openat(AT_FDCWD, dirname, O_DIRECTORY|O_NONBLOCK|O_cloexec|O_SEARCH)) < 0)
+	#else
+			if((n = open(dirname, O_DIRECTORY|O_cloexec|O_SEARCH)) < 0)  /* vulnerable to race, but that's not a concern */
+	#endif
+				return 1;  /* couldn't even obtain a file descriptor */
+			if(fchdir(n) < 0)
+				return 1;  /* couldn't change the directory (perhaps Linux <3.2.23) */
+			return 0;
+		}
+		else if(child==-1)
+		{
+			rmdir(dirname);
+			return 1;
+		}
+		/* a parent process removes the empty dir because that's safer than 'cd ..' */
+		waitpid(child, &n, 0);
+		rmdir(dirname);
+		return WEXITSTATUS(n);
+	}
+}end
+
+tst	openat_enotdir note{ does openat set ENOTDIR when it fails because the file isn't a directory }end output{
+	#include <ast.h>
+	#include <stdio.h>
+	#include <errno.h>
+	#if !_lib_openat
+	#error openat(2) doesn't exist, so this test is pointless
+	#endif
+	int main(void)
+	{
+		char	filename[64];
+		int	n, saverrno;
+		pid_t	child;
+		sprintf(filename,".file.%u",(unsigned int)getpid());
+		open(filename,O_RDWR|O_CREAT);  /* let the OS close the fd after completion */
+		openat(AT_FDCWD, filename, O_DIRECTORY|O_NONBLOCK|O_cloexec|O_SEARCH);
+		saverrno = errno;
+		unlink(filename);
+		if(saverrno != ENOTDIR)
+			return 1;
+		return 0;
+	}
+}end

src/cmd/ksh93/include/io.h

@@ -26,6 +26,7 @@
 
 #include	<ast.h>
 #include	<sfio.h>
+#include	"FEATURE/fchdir"
 
 #ifndef IOBSIZE
 #   define  IOBSIZE	(SFIO_BUFSIZE*sizeof(char*))
@@ -52,6 +53,12 @@
     struct ionod;
 #endif /* !ARG_RAW */
 
+/* if O_SEARCH/O_PATH is unreliable for fchdir, it's not worth using */
+#if !_fchdir_osearch_compat
+#undef O_SEARCH
+#define O_SEARCH 0
+#endif
+
 /*
  * Check if there is an editor active while avoiding repetitive #if flaggery.
  * The 0 definition is used to optimize out code if no editor is compiled in.