config: do not ungetc EOF
PATCH 1/1 config: do not ungetc EOF
When we are parsing a config value, if we see a carriage return, we fgetc the next character to see if it is a line feed (in which case we silently drop the CR). If it isn't, we then ungetc the character, and take the literal CR.
But we never check whether we in fact got a character at all. If the config file ends in CR, we will get EOF here, and try to ungetc EOF. This works OK for a real stdio stream. The ungetc returns an error, and the next fgetc will then return EOF again.
However, our custom buffer-based stream is not so fortunate. It happily rewinds the position of the stream by one character, ignoring the fact that we fed it EOF. The next fgetc call returns the final CR again, over and over, and we end up in an infinite loop.
---
Looks like this weirdness has been there for a long time, but we only added the git config --blob code in v1.8.4.
I also notice that config_buf_ungetc does not actually ungetc the character we give it; it just rewinds one character in the stream. This is fine, because we always feed the last-retrieved character. I dunno if it is worth fixing (it also would have fixed this infinite loop, but for the wrong reason; we would have stuck "-1" back into the stream, and retrieved it on the next fgetc rather than the same '\r' over and over).
config.c | 3 ++- t/t1307-config-blob.sh | 9 +++++++++ 2 files changed, 11 insertions(+), 1 deletion(-)
config.c +2 -1
@@ -235,7 +235,8 @@ static int get_next_char(void)/* DOS like systems */ c = cf->do_fgetc (cf);if (c !='\n' ) { - cf->do_ungetc (c, cf); +if (c !=EOF ) + cf->do_ungetc (c, cf); c ='\r' ; } }
t/t1307-config-blob.sh +9 -0
@@ -67,4 +67,13 @@ test_expect_success 'parse errors in blobs are properly attributed' ' grep "HEAD:config" err ' +test_expect_success 'can parse blob ending with CR' ' + printf "[some]key = value\\r" >config && + git add config && + git commit -m CR && + echo value >expect && + git config --blob=HEAD:config some.key >actual && + test_cmp expect actual +' + test_done
Signature · 1 line
2.3.0.rc1.287.g761fd19
Quoted 6 lines · Jeff King
I also notice that
config_buf_ungetcdoes not actually ungetc the character we give it; it just rewinds one character in the stream. This is fine, because we always feed the last-retrieved character. I dunno if it is worth fixing (it also would have fixed this infinite loop, but for the wrong reason; we would have stuck "-1" back into the stream, and retrieved it on the next fgetc rather than the same '\r' over and over).
Here's a patch to deal with that. I'm not sure if it's worth doing or not.
-- >8 --
Subject: [PATCH] config_buf_ungetc: warn when pushing back a random character
Our config code simulates a stdio stream around a buffer, but our fake ungetc() does not behave quite like the real one. In particular, we only rewind the position by one character, but do _not_ actually put the character from the caller into position.
It turns out that this does not matter, because we only ever push back the character we just read. In other words, such an assignment would be a noop. But because the function is called ungetc, and because it takes a character parameter, it is a mistake waiting to happen.
Actually assigning the character into the buffer would be ideal, but our pointer is actually a "const" copy of the buffer. We do not know who the real owner of the buffer is in this code, and would not want to munge their contents.
Instead, we can simply add an assertion that matches what the current caller does, and will let us know if new callers are added that violate the contract.
--- config.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
config.c +6 -2
@@ -73,8 +73,12 @@ static int config_buf_fgetc(struct config_source *conf)static int config_buf_ungetc (int c,struct config_source *conf) { -if (conf->u.buf.pos >0 ) -return conf->u.buf.buf[--conf->u.buf.pos]; +if (conf->u.buf.pos >0 ) { + conf->u.buf.pos--; +if (conf->u.buf.buf[conf->u.buf.pos] != c) +die ("BUG: config_buf can only ungetc the same character" ); +return c; + }return EOF ; }
Signature · 1 line
2.3.0.rc1.287.g761fd19
Quoted 11 lines · Jeff King
Quoted 6 lines · Jeff King
I also notice that
config_buf_ungetcdoes not actually ungetc the character we give it; it just rewinds one character in the stream. This is fine, because we always feed the last-retrieved character. I dunno if it is worth fixing (it also would have fixed this infinite loop, but for the wrong reason; we would have stuck "-1" back into the stream, and retrieved it on the next fgetc rather than the same '\r' over and over).Here's a patch to deal with that. I'm not sure if it's worth doing or not.
I am not sure, either. If this were to become stdio emulator over random in-core data used throughout the system, perhaps.
But in its current form it is tied to the implementation of config.c very strongly, so...
Quoted 48 lines
-- >8 --
Subject: [PATCH]config_buf_ungetc: warn when pushing back a random characterOur config code simulates a stdio stream around a buffer, but our fake
ungetc()does not behave quite like the real one. In particular, we only rewind the position by one character, but do _not_ actually put the character from the caller into position.It turns out that this does not matter, because we only ever push back the character we just read. In other words, such an assignment would be a noop. But because the function is called ungetc, and because it takes a character parameter, it is a mistake waiting to happen.
Actually assigning the character into the buffer would be ideal, but our pointer is actually a "const" copy of the buffer. We do not know who the real owner of the buffer is in this code, and would not want to munge their contents.
Instead, we can simply add an assertion that matches what the current caller does, and will let us know if new callers are added that violate the contract.
Signed-off-by Jeff King <[email protected]>--- config.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)config.c +6
-2@@ -73,8 +73,12 @@ static int config_buf_fgetc(struct config_source *conf)static int config_buf_ungetc (int c,struct config_source *conf) { -if (conf->u.buf.pos >0 ) -return conf->u.buf.buf[--conf->u.buf.pos]; +if (conf->u.buf.pos >0 ) { + conf->u.buf.pos--; +if (conf->u.buf.buf[conf->u.buf.pos] != c) +die ("BUG: config_buf can only ungetc the same character" ); +return c; + }return EOF ; }
Quoted 11 lines · Junio C Hamano
Quoted 9 lines · Jeff King
Quoted 6 lines · Jeff King
I also notice that
config_buf_ungetcdoes not actually ungetc the character we give it; it just rewinds one character in the stream. This is fine, because we always feed the last-retrieved character. I dunno if it is worth fixing (it also would have fixed this infinite loop, but for the wrong reason; we would have stuck "-1" back into the stream, and retrieved it on the next fgetc rather than the same '\r' over and over).
Quoted 8 lines
Quoted 2 lines
Here's a patch to deal with that. I'm not sure if it's worth doing or not.
I am not sure, either. If this were to become stdio emulator over random in-core data used throughout the system, perhaps.
But in its current form it is tied to the implementation of
config.cvery strongly, so...
Yeah, that was my thinking, and why I have doubts. Maybe a comment would make more sense, like the patch below. I am also OK with just leaving it as-is.
-- >8 --
Subject: [PATCH] config_buf_ungetc: document quirks in a comment
Our config_buf_ungetc implements just enough for the config code to work. That's OK, but we would not want anyone to mistakenly move it elsewhere as a general purpose ungetc.
--- config.c | 6 ++++++ 1 file changed, 6 insertions(+)
config.c +6 -0
@@ -71,6 +71,12 @@ static int config_buf_fgetc(struct config_source *conf)return EOF ; } +/* +* Note that this is not a real ungetc replacement. It only rewinds +* the position, and ignores the "c" parameter, rather than +* putting it into our (const) buffer. That's good enough for +* the callers here, though. +*/ static int config_buf_ungetc (int c,struct config_source *conf) {if (conf->u.buf.pos >0 )
Signature · 1 line
2.3.0.rc1.287.g761fd19
Quoted 22 lines · Jeff King
Quoted 11 lines · Junio C Hamano
Quoted 9 lines · Jeff King
Quoted 6 lines · Jeff King
I also notice that
config_buf_ungetcdoes not actually ungetc the character we give it; it just rewinds one character in the stream. This is fine, because we always feed the last-retrieved character. I dunno if it is worth fixing (it also would have fixed this infinite loop, but for the wrong reason; we would have stuck "-1" back into the stream, and retrieved it on the next fgetc rather than the same '\r' over and over).Quoted 8 lines
Quoted 2 lines
Here's a patch to deal with that. I'm not sure if it's worth doing or not.
I am not sure, either. If this were to become stdio emulator over random in-core data used throughout the system, perhaps.
But in its current form it is tied to the implementation of
config.cvery strongly, so...
Quoted 6 lines
Yeah, that was my thinking, and why I have doubts. Maybe a comment would make more sense, like the patch below. I am also OK with just leaving it as-is.
-- >8 --
Subject: [PATCH]config_buf_ungetc: document quirks in a comment
I think a comment would be fine as well. Both helps to quickly find the cause why our ungetc() might not behave as the caller expects. But I think one of both would be good so we document that this behavior is in fact intentional.
Cheers Heiko
Quoted 9 lines · Jeff King
Quoted 6 lines · Jeff King
I also notice that
config_buf_ungetcdoes not actually ungetc the character we give it; it just rewinds one character in the stream. This is fine, because we always feed the last-retrieved character. I dunno if it is worth fixing (it also would have fixed this infinite loop, but for the wrong reason; we would have stuck "-1" back into the stream, and retrieved it on the next fgetc rather than the same '\r' over and over).
Quoted 5 lines
Here's a patch to deal with that. I'm not sure if it's worth doing or not.
-- >8 --
Subject: [PATCH]config_buf_ungetc: warn when pushing back a random character
Thanks for noticing and fixing both. I think it is worth adding this assertion. If someone in the future comes along and uses our fake ungetc() wrong it might save some trouble figuring out whats wrong.
Cheers Heiko