BufReader: In Seek impl, remove extra discard_buffer call

[czipperz]

As far as I can tell, this code does nothing. I'm not sure why it even is there.

[rust-highfive]

r? @nikomatsakis

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[czipperz]

Since this will be peer reviewed, I might as well ask: should this refactored to not use a result variable? It would simplify the code but would require storing the pos and cap fields locally (discard_buffer resets them).

[czipperz]

It seems this was introduced in the original implementation. It's been dead code for a while.

[taiki-e]

I tried to do the same when I added async BufReader in futures-rs, but that was not correct (rust-lang/futures-rs#1573 (comment)).

There is no Pending in std::io, but there is a case to retry when ErrorKind::Interrupted is returned, so I think the same problem will occur when called in the order Ok - Err(ErrorKind::Interrupted).

[czipperz]

Mmh so it's because of the ? on the next line. Shoot

[czipperz]

I'm going to add try to add a test so this behavior is ensured.

[taiki-e]

it's because of the ? on the next line.

Yeah, after early return with the next line's ? operator, enter the same branch again and call inner.seek(SeekFrom::Current(-remainder))?. Since we are calling inner.seek(SeekFrom::Current(-remainder)) twice at this point, the next line's inner.seek(SeekFrom::Current(n))? will seek the wrong position.

(If pos and cap are reset with discard_buffer, then remainder will be 0 at the next time, so it will not enter this branch.)

[czipperz]

Cool. I undid my commit and added a test that fails without that line.

[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:10dfcfe8:start=1558814688581140561,finish=1558814777780621821,duration=89199481260
$ git checkout -qf FETCH_HEAD
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Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
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Check compiletest suite=codegen mode=codegen (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:19:51] 
[01:19:51] running 143 tests
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[01:19:55] test result: ok. 113 passed; 0 failed; 30 ignored; 0 measured; 0 filtered out
[01:19:55] 
[01:19:55]  finished in 4.598
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[01:19:57] running 9 tests
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Check compiletest suite=debuginfo mode=debuginfo-gdb+lldb (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:20:13] 
[01:20:13] running 122 tests
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[01:32:10] travis_fold:start:test_stage1-std
travis_time:start:test_stage1-std
Testing std stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:32:10]    Compiling std v0.0.0 (/checkout/src/libstd)
[01:32:31] error: function is never used: `test_buffered_reader_seek_underflow_discard_buffer_between_seeks`
[01:32:31]      |
[01:32:31]      |
[01:32:31] 1165 |     fn test_buffered_reader_seek_underflow_discard_buffer_between_seeks() {
[01:32:31]      |
[01:32:31]      = note: `-D dead-code` implied by `-D warnings`
[01:32:31] 
[01:32:32] error: aborting due to previous error
[01:32:32] error: aborting due to previous error
[01:32:32] 
[01:32:32] error: Could not compile `std`.
[01:32:32] 
[01:32:32] To learn more, run the command again with --verbose.
[01:32:32] 
[01:32:32] 
[01:32:32] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace compiler-builtins-c" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "-p" "std" "--" "--quiet"
[01:32:32] 
[01:32:32] 
[01:32:32] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:32:32] Build completed unsuccessfully in 0:24:34
[01:32:32] Build completed unsuccessfully in 0:24:34
[01:32:32] Makefile:48: recipe for target 'check' failed
[01:32:32] make: *** [check] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:1e28b616
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Sat May 25 21:38:59 UTC 2019
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travis_time:start:240454ce
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
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travis_time:start:0366f060
travis_time:start:0366f060
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
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travis_time:start:0fca9e4c
$ dmesg | grep -i kill

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[nikomatsakis]

@bors r+ rollup -- thanks!

[bors]

📌 Commit b1ae49c has been approved by nikomatsakis