Create README for libpod

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 opencontainers/runc PR containers#162:

-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:

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 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 <[email protected]>

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.

[v2: skip patching (*Builder).Run]

For more details, a quote from opencontainers/runc PR containers#162:

-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:

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 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 <[email protected]>

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.

[v2: skip patching (*Builder).Run]

For more details, a quote from opencontainers/runc PR containers#162:

-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:

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 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 <[email protected]>

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.

[v2: skip patching (*Builder).Run]

For more details, a quote from opencontainers/runc PR containers#162:

-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:

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

-end-quote-

Signed-off-by: Kir Kolyshkin <[email protected]>
Signed-off-by: Daniel J Walsh <[email protected]>

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.

[v2: skip patching (*Builder).Run]

For more details, a quote from opencontainers/runc PR containers#162:

-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:

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

-end-quote-

Signed-off-by: Kir Kolyshkin <[email protected]>
Signed-off-by: Daniel J Walsh <[email protected]>