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libc: Fix fopen with mode "r"

On all supported platforms, O_RDONLY is 0, so flags & O_RDONLY is
never true and the FILE does not get marked as _IOREAD, causing all
read operations to fail with EBADF.

To fix this, introduce O_ACCMODE in sys.h, containing a mask of the
bits used for the file access mode. Then, in _fpopen, switch on
access mode.

--- a/include/bits/darwin/sys.h

+++ b/include/bits/darwin/sys.h

@@ -1,6 +1,7 @@

 #define O_RDONLY  0x00000000

 #define O_WRONLY  0x00000001

 #define O_RDWR    0x00000002

+#define O_ACCMODE 0x00000003

 #define O_TRUNC   0x00000400

 #define O_APPEND  0x00000008

--- a/include/bits/dragonfly/sys.h

+++ b/include/bits/dragonfly/sys.h

@@ -1,6 +1,7 @@

 #define O_RDONLY  0x00000000

 #define O_WRONLY  0x00000001

 #define O_RDWR    0x00000002

+#define O_ACCMODE 0x00000003

 #define O_TRUNC   0x00000400

 #define O_APPEND  0x00000008

--- a/include/bits/linux/sys.h

+++ b/include/bits/linux/sys.h

@@ -1,6 +1,7 @@

 #define O_RDONLY  0x00000000

 #define O_WRONLY  0x00000001

 #define O_RDWR    0x00000002

+#define O_ACCMODE 0x00000003

 #define O_TRUNC   0x00000200

 #define O_APPEND  0x00000400

--- a/include/bits/netbsd/sys.h

+++ b/include/bits/netbsd/sys.h

@@ -1,6 +1,7 @@

 #define O_RDONLY  0x00000000

 #define O_WRONLY  0x00000001

 #define O_RDWR    0x00000002

+#define O_ACCMODE 0x00000003

 #define O_TRUNC   0x00000400

 #define O_APPEND  0x00000008

--- a/include/bits/openbsd/sys.h

+++ b/include/bits/openbsd/sys.h

@@ -1,6 +1,7 @@

 #define O_RDONLY  0x00000000

 #define O_WRONLY  0x00000001

 #define O_RDWR    0x00000002

+#define O_ACCMODE 0x00000003

 #define O_TRUNC   0x00000400

 #define O_APPEND  0x00000008

--- a/src/libc/stdio/_fpopen.c

+++ b/src/libc/stdio/_fpopen.c

@@ -63,12 +63,17 @@

 	if (!bin)

 		fp->flags |= _IOTXT;

-	if (flags & O_RDWR)

+	switch (flags & O_ACCMODE) {

+	case O_RDWR:

 		fp->flags |= _IORW;

-	else if (flags & O_RDONLY)

+		break;

+	case O_RDONLY:

 		fp->flags |= _IOREAD;

-	else

+		break;

+	case O_WRONLY:

 		fp->flags |= _IOWRITE;

+		break;

+	}

 	fp->lp = fp->rp = fp->wp = NULL;