Simplify and fix os.MkdirAll() usage #162
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TL;DR: check for IsExist(err) after a failed MkdirAll() is both
redundant and wrong -- so two reasons to remove it.
Quoting MkdirAll documentation:
> MkdirAll creates a directory named path, along with any necessary
> parents, and returns nil, or else returns an error. If path
> is already a directory, MkdirAll does nothing and returns nil.
This means two things:
1. If a directory to be created already exists, no error is
returned.
2. If the error returned is IsExist (EEXIST), it means there exists
a non-directory with the same name as MkdirAll need to use for
directory. Example: we want to MkdirAll("a/b"), but file "a"
(or "a/b") already exists, so MkdirAll fails.
The above is a theory, based on quoted documentation and my UNIX
knowledge.
3. In practice, though, current MkdirAll implementation [1] returns
ENOTDIR in most of cases described in #2, with the exception when
there is a race between MkdirAll and someone else creating the
last component of MkdirAll argument as a file. In this very case
MkdirAll() will indeed return EEXIST.
Because of #1, IsExist check after MkdirAll is not needed.
Because of #2 and #3, ignoring IsExist error is just plain wrong,
as directory we require is not created. It's cleaner to report
the error now.
Note this error is all over the tree, I guess due to copy-paste,
or trying to follow the same usage pattern as for Mkdir(),
or some not quite correct examples on the Internet.
[1] https://github.com/golang/go/blob/f9ed2f75/src/os/path.go
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
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Here's an example |
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@kolyshkin You're right, I actually don't see how it can return |
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@LK4D4 It actually can return EEXIST in a case of the following race (which I was lazy to explain in details, but since you asked for it...). First, MkdirAll calls stat() and gets ENOENT, so it goes on to check and create all the parents. At this time, someone else creates a file under the same name as the last MkdirAll component. Finally, MkdirAll calls Mkdir to create that last component, Mkdir returns EEXIST which MkdirAll passes to the caller. |
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@kolyshkin Totally, thanks for explanation! |
LK4D4
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Jul 30, 2015
Simplify and fix os.MkdirAll() usage
stefanberger
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Sep 8, 2017
*.md: markdown formatting
kolyshkin
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May 16, 2020
This subtle bug keeps lurking in because error checking for `Mkdir()` and `MkdirAll()` is slightly different wrt `EEXIST`/`IsExist`: - for `Mkdir()`, `IsExist` error should (usually) be ignored (unless you want to make sure directory was not there before) as it means "the destination directory was already there"; - for `MkdirAll()`, `IsExist` error should NEVER be ignored. This commit removes ignoring the IsExist error, as it should not be ignored. For more details, a quote from my opencontainers/runc#162 (July 2015): -quote- TL;DR: check for IsExist(err) after a failed MkdirAll() is both redundant and wrong -- so two reasons to remove it. Quoting MkdirAll documentation: MkdirAll creates a directory named path, along with any necessary parents, and returns nil, or else returns an error. If path is already a directory, MkdirAll does nothing and returns nil. This means two things: If a directory to be created already exists, no error is returned. If the error returned is IsExist (EEXIST), it means there exists a non-directory with the same name as MkdirAll need to use for directory. Example: we want to MkdirAll("a/b"), but file "a" (or "a/b") already exists, so MkdirAll fails. The above is a theory, based on quoted documentation and my UNIX knowledge. In practice, though, current MkdirAll implementation [1] returns ENOTDIR in most of cases described in containers#2, with the exception when there is a race between MkdirAll and someone else creating the last component of MkdirAll argument as a file. In this very case MkdirAll() will indeed return EEXIST. Because of containers#1, IsExist check after MkdirAll is not needed. Because of containers#2 and containers#3, ignoring IsExist error is just plain wrong, as directory we require is not created. It's cleaner to report the error now. Note this error is all over the tree, I guess due to copy-paste, or trying to follow the same usage pattern as for Mkdir(), or some not quite correct examples on the Internet. [1] https://github.com/golang/go/blob/f9ed2f75/src/os/path.go -end-quote- Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
This was referenced May 16, 2020
kolyshkin
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May 18, 2020
This subtle bug keeps lurking in because error checking for `Mkdir()` and `MkdirAll()` is slightly different wrt `EEXIST`/`IsExist`: - for `Mkdir()`, `IsExist` error should (usually) be ignored (unless you want to make sure directory was not there before) as it means "the destination directory was already there"; - for `MkdirAll()`, `IsExist` error should NEVER be ignored. This commit removes ignoring the IsExist error, as it should not be ignored. For more details, a quote from my opencontainers/runc#162 (July 2015): -quote- TL;DR: check for IsExist(err) after a failed MkdirAll() is both redundant and wrong -- so two reasons to remove it. Quoting MkdirAll documentation: MkdirAll creates a directory named path, along with any necessary parents, and returns nil, or else returns an error. If path is already a directory, MkdirAll does nothing and returns nil. This means two things: If a directory to be created already exists, no error is returned. If the error returned is IsExist (EEXIST), it means there exists a non-directory with the same name as MkdirAll need to use for directory. Example: we want to MkdirAll("a/b"), but file "a" (or "a/b") already exists, so MkdirAll fails. The above is a theory, based on quoted documentation and my UNIX knowledge. In practice, though, current MkdirAll implementation [1] returns ENOTDIR in most of cases described in containers#2, with the exception when there is a race between MkdirAll and someone else creating the last component of MkdirAll argument as a file. In this very case MkdirAll() will indeed return EEXIST. Because of containers#1, IsExist check after MkdirAll is not needed. Because of containers#2 and containers#3, ignoring IsExist error is just plain wrong, as directory we require is not created. It's cleaner to report the error now. Note this error is all over the tree, I guess due to copy-paste, or trying to follow the same usage pattern as for Mkdir(), or some not quite correct examples on the Internet. [1] https://github.com/golang/go/blob/f9ed2f75/src/os/path.go -end-quote- Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
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Please review; I find it moderately serious as missing error from MkdirAll() can lead to some other errors at a later stage, possibly hard to track down to the cause.