RFR: 8286160: (fs) Files.exists returns unexpected results with C:\pagefile.sys because it's not readable [v2]

[Brian Burkhalter]

On Wed, 8 Jun 2022 00:21:21 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

>> If in `WindosFileSystemProvider.checkAccess(Path obj, AccessMode... modes)` the sub-test `checkReadAccess(WindowsPath)` fails and no modes are specified, then try to determine file existence by reading the file's attributes.
>
> Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8286160: Move check for existence with no modes specifieds

> > [D]id you do any performance checking compared to the existing code?
> 
> No I did not, at least not yet.

Performance measurements for the before and after are as follows:

**Existing Code**

Benchmark                        (name)   Mode  Cnt      Score      Error  Units

CheckAccess.exists      C:\pagefile.sys  thrpt   10  10525.872 ±  247.734  ops/s
CheckAccess.exists                   C:  thrpt   10  11489.428 ±   69.678  ops/s
CheckAccess.exists                 junk  thrpt   10  15368.786 ± 2322.512  ops/s

CheckAccess.isReadable  C:\pagefile.sys  thrpt   10   9674.403 ±  271.966  ops/s
CheckAccess.isReadable               C:  thrpt   10  10578.904 ±  201.453  ops/s
CheckAccess.isReadable             junk  thrpt   10  16072.720 ±  117.358  ops/s

CheckAccess.isWritable  C:\pagefile.sys  thrpt   10   7211.853 ±   62.367  ops/s
CheckAccess.isWritable               C:  thrpt   10    919.860 ±   13.846  ops/s
CheckAccess.isWritable             junk  thrpt   10    832.815 ±   49.616  ops/s

 **Modified Code**

Benchmark                        (name)   Mode  Cnt      Score     Error  Units
CheckAccess.exists      C:\pagefile.sys  thrpt   10  11412.256 ± 710.767  ops/s
CheckAccess.exists                   C:  thrpt   10  28780.641 ± 345.568  ops/s
CheckAccess.exists                 junk  thrpt   10  28277.019 ± 219.792  ops/s

CheckAccess.isReadable  C:\pagefile.sys  thrpt   10  10465.858 ± 176.803  ops/s
CheckAccess.isReadable               C:  thrpt   10  11343.234 ± 200.306  ops/s
CheckAccess.isReadable             junk  thrpt   10  17053.404 ±  96.515  ops/s

CheckAccess.isWritable  C:\pagefile.sys  thrpt   10   7713.395 ±  89.954  ops/s
CheckAccess.isWritable               C:  thrpt   10    989.610 ±   5.155  ops/s
CheckAccess.isWritable             junk  thrpt   10    873.649 ±   6.802  ops/s

The updated code shows higher throughput for all tests, and substantially higher for a couple of the `exists()` test.

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PR: https://git.openjdk.java.net/jdk/pull/9045