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Don't fail in gz_load() if read would block #993

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Don't fail in gz_load() if read would block #993

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3e31e7b
Don't fail in gz_load() if read would block
Jul 18, 2024
f16f656
Handle EWOULDBLOCK/EAGAIN in gz_load() call from gz_read
Jul 20, 2024
adb2b5b
More handling of EWOULDBLOCK/EAGAIN results in calls from gz_read
Jul 20, 2024
6e8ddfd
In gz_read(), do not introduce blocked read state after gz_fetch() un…
Jul 21, 2024
3ddf538
Add non-blocking socket test code; first draft, no formal test yet
Jul 23, 2024
12eca61
Rename test program
Jul 23, 2024
1cf7557
Build GZ-non-blocking binary
Jul 23, 2024
d781605
Add fork of client by server in test/gznonblk.c; not done yet
Jul 23, 2024
73b57d4
Remove [int s]; use '-stopserver-' to stop server
Jul 23, 2024
a9f3755
Fix post-fork(2) bug; clean up comments
Jul 23, 2024
ea6e677
test/gznonblk.c now working, server spawns client and reports exit st…
Jul 24, 2024
f6a01bb
Update Makefile.in to run test of gznonblk
Jul 24, 2024
880c261
Add /gznonblk to .gitignore
Jul 24, 2024
6366659
Add dependencies for gznonblk* to Makefile.in
Jul 24, 2024
5c75fcb
Move testgznonblk dependency to the right place in Makefile.in
Jul 24, 2024
5ab0725
test/gznonblk.c: add more comments, rearrange logging
Jul 24, 2024
511989d
Fix typo
Jul 24, 2024
970385d
Add usage options (--help[-long]) to test/gznonblk.c
Jul 24, 2024
903c3ab
Clean up test/gznonblk.c Usage
Jul 24, 2024
c6152bb
test/gznonblk.c: add header comments
Jul 24, 2024
df4df93
Update ChangeLog
drbitboy Jul 25, 2024
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12 changes: 10 additions & 2 deletions gzread.c
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Original file line number Diff line number Diff line change
Expand Up @@ -24,7 +24,7 @@ local int gz_load(gz_statep state, unsigned char *buf, unsigned len,
break;
*have += (unsigned)ret;
} while (*have < len);
if (ret < 0) {
if (ret < 0 && errno != EAGAIN && errno != EWOULDBLOCK) {
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@drbitboy drbitboy Jul 22, 2024 •
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This is the core of this PR, because all of the read-related wrappers here in gzread.c
end up calling this gz_load routine: when

  • BOTH state-fd (file descriptor) is non-blocking (fcntl(fd, SET_FL, fcntl(fd, GET_FL) | O_NONBLOCK))
  • AND there are no data available to read,

then the read(2) return value (ret above, assigned on line 22) will be -1, and errno will be either EWOULDBLOCK or EAGAIN (Resource not available).

The change here prevents that case from returning -1 from gz_load() here, which in turn
allows the calling routine* to continue working with data from the file descriptor.

It gets a little tricky, because no bytes are returned, and 0 as the return value from read(2)
indicates EOF (End-Of-File).

N.B. tt may make sense to clear errno to a value of 0 here in gz_load(), just before line 22 above.

* gz_avail() or gz_fetch() or gz_read(), with the latter being the ultimate caller of the first two.

gz_error(state, Z_ERRNO, zstrerror());
return -1;
}
Expand Down Expand Up @@ -268,6 +268,7 @@ local int gz_skip(gz_statep state, z_off64_t len) {
local z_size_t gz_read(gz_statep state, voidp buf, z_size_t len) {
z_size_t got;
unsigned n;
int load_errno;
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Variable to keep track of errno from the read(2) in gz_load(), whether gz_load() is called from here (gz_read())

  • EITHER directly,
  • OR indirectly via gz_decomp() or gz_fetch().


/* if len is zero, avoid unnecessary operations */
if (len == 0)
Expand All @@ -283,6 +284,7 @@ local z_size_t gz_read(gz_statep state, voidp buf, z_size_t len) {
/* get len bytes to buf, or less than len if at the end */
got = 0;
do {
load_errno = 0;
/* set n to the maximum amount of len that fits in an unsigned int */
n = (unsigned)-1;
if (n > len)
Expand All @@ -307,25 +309,31 @@ local z_size_t gz_read(gz_statep state, voidp buf, z_size_t len) {
buffer */
else if (state->how == LOOK || n < (state->size << 1)) {
/* get more output, looking for header if required */
errno = 0;
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Prepare for errno to become EAGAIN/EWOULDBLOCK, indicating that the read(2) on the
non-blocking file descriptor would block, so the read(2) returns a value of -1.

if (gz_fetch(state) == -1)
return 0;
if (!state->x.have) { load_errno = errno; }
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The call to gz_fetch() (line 313may have resulted in a call to gz_load()/read(2) and updated the internal output buffer, so errno may have a value of EAGAIN/EWOULDBLOCK, indicating that read(2) would have blocked on a non-blocking file.

The next line, 316, will continue to the while at the end of this do-while loop.

The state->x internal output buffer is encapsulated in state->x.next and state->x.have (see lines 293-300 above).

So if there is any data in the internal output buffer (state->x.have is non-zero), then don't copy the value, possibly EAGAIN/EWOULDBLOCK, of errno from gz_fetch() here, because that would cause the while to exit the loop and the data in the internal input buffer would be lost.

If the the state->x internal output buffer is empty here (state->x.have is 0), then there was no data read in the gz_fetch() call (line 313), and it is save to copy the errno value to load_errno and continue, which, if errno is EAGAIN/EWOULDBLOCK here, will exit this do-while loop .

continue; /* no progress yet -- go back to copy above */
/* the copy above assures that we will leave with space in the
output buffer, allowing at least one gzungetc() to succeed */
}

/* large len -- read directly into user buffer */
else if (state->how == COPY) { /* read directly */
errno = 0;
if (gz_load(state, (unsigned char *)buf, n, &n) == -1)
return 0;
load_errno = errno;
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gz_load() (line 324) read the data directly into the external output buffer (buf), and state->x was empty and not used here, so put the value of errno into load_errno, which if that value is EAGAIN/EWOULDBLOCK, will cause the do-while loop to exit (line 346).

}

/* large len -- decompress directly into user buffer */
else { /* state->how == GZIP */
state->strm.avail_out = n;
state->strm.next_out = (unsigned char *)buf;
errno = 0;
if (gz_decomp(state) == -1)
return 0;
load_errno = errno;
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gz_decomp() (line 334) read the data directly into the external output buffer (buf; lines 331-332), and that buffer will be declared empty when state->x.have is cleared to 0 (line 338), so put the value of errno into load_errno, which if the value is ...

n = state->x.have;
state->x.have = 0;
}
Expand All @@ -335,7 +343,7 @@ local z_size_t gz_read(gz_statep state, voidp buf, z_size_t len) {
buf = (char *)buf + n;
got += n;
state->x.pos += n;
} while (len);
} while (len && load_errno != EAGAIN && load_errno != EWOULDBLOCK);
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Execute another pass on this do-while loop if

  • BOTH there is room left in the output buffer
  • AND we did not execute a blocking read on a non-blocking input on this pass


/* return number of bytes read into user buffer */
return got;
Expand Down