awk: fix truncated input after fflush

Before the "native" awk work, a call to the fflush function resulted
in one or more calls to the APE fflush(2).

Calling fflush on a stream open for reading has different behavior
based on the environment: within APE, it's a no-op¹; on OpenBSD, it's
an error²; in musl, it depends on whether or not the underlying file
descriptor is seekable³; etc. I'm sure glibc is subtly different.

Now that awk uses libbio, things are different: calling Bflush(2) on a
file open for reading simply discards any data in the buffer. This
explains why we're seeing truncated input. When awk attempts to read
in the next record, there's nothing in the buffer and no more data to
read so it gets EOF and exits normally. Note that this behavior is not
documented in bio(2). It was added in the second edition but I haven't
figured out why or what depends on it.

The simple fix is to have awk only call Bflush on files that were
opened for writing. You could argue that this is the only correct
behavior according to the awk(1) manual and it is, in fact, how GNU
awk behaves⁴.

1. /sys/src/ape/lib/ap/stdio/fflush.c
2. https://cvsweb.openbsd.org/src/lib/libc/stdio/fflush.c?rev=1.9
3. https://git.musl-libc.org/cgit/musl/tree/src/stdio/fflush.c
4. https://git.savannah.gnu.org/cgit/gawk.git/tree/io.c#n1492

1 files changed, 6 insertions, 4 deletions

diff --git a/sys/src/cmd/awk/run.c b/sys/src/cmd/awk/run.c
index 9af1b4b01..94918a29f 100644
--- a/sys/src/cmd/awk/run.c
+++ b/sys/src/cmd/awk/run.c
@@ -1706,6 +1706,7 @@ void stdinit(void)	/* in case stdin, etc., are not constants */
 	files[1].fp = &stdout;
 	files[2].fp = &stderr;
 }
+#define writing(m) ((m) != LT && (m) != LE)
 
 Biobuf *openfile(int a, char *us)
 {
@@ -1719,8 +1720,11 @@ Biobuf *openfile(int a, char *us)
 		if (files[i].fname && strcmp(s, files[i].fname) == 0) {
 			if (a == files[i].mode || (a==APPEND && files[i].mode==GT))
 				return files[i].fp;
-			if (a == FFLUSH)
+			if (a == FFLUSH) {
+				if(!writing(files[i].mode))
+					return nil;
 				return files[i].fp;
+			}
 		}
 	if (a == FFLUSH)	/* didn't find it, so don't create it! */
 		return nil;
@@ -1813,12 +1817,10 @@ void closeall(void)
 void flush_all(void)
 {
 	int i;
-
 	for (i = 0; i < FOPEN_MAX; i++)
-		if (files[i].fp)
+		if (files[i].fp && writing(files[i].mode))
 			Bflush(files[i].fp);
 }
-
 void backsub(char **pb_ptr, char **sptr_ptr);
 
 Cell *sub(Node **a, int)	/* substitute command */