RFR: 8304020: Speed up test/jdk/java/util/zip/ZipFile/TestTooManyEntries.java and clarify its purpose [v10]

[Jaikiran Pai]

On Mon, 30 Oct 2023 15:50:49 GMT, Eirik Bjorsnos <duke at openjdk.org> wrote:

>> Please review this PR which speeds up TestTooManyEntries and clarifies its purpose:
>> 
>> - The name 'TestTooManyEntries' does not clearly convey the purpose of the test. What is tested is the validation that the total CEN size fits in a Java byte array. Suggested rename: CenSizeTooLarge
>> - The test creates DEFLATED entries which incurs zlib costs and File Data / Data Descriptors for no additional benefit. We can use STORED instead.
>> - By creating a single LocalDateTime and setting it with `ZipEntry.setTimeLocal`, we can avoid repeated time zone calculations. 
>> - The name of entries is generated by calling UUID.randomUUID, we could use simple counter instead.
>> - The produced file is unnecessarily large. We know how large a CEN entry is, let's take advantage of that to create a file with the minimal size.
>> - By adding a maximally large extra field to the CEN entries, we get away with fewer CEN records and save memory
>> - The summary and comments of the test can be improved to help explain the purpose of the test and how we reach the limit being tested.
>> - By writing sparse 'holes' until the last CEN entry, we can reduce required disk space.
>> 
>> These speedups reduced the runtime from 4 min 17 sec to 3 seconds on my Macbook Pro. The produced ZIP size was reduced from 5.7 GB to ~4K. Memory consumption is down from 8GB to something like 12MB.
>
> Eirik Bjorsnos has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Replace the 'afterLastCEN' boolean with a 'sparse' state variable

test/jdk/java/util/zip/ZipFile/CenSizeTooLarge.java line 204:

> 202:                 channel.position(position);
> 203:                 // Check for last CEN record
> 204:                 if (Arrays.equals(LAST_CEN_COMMENT_BYTES, 0, LAST_CEN_COMMENT_BYTES.length, b, off, len)) {

The way the instance of a `SparseOutputStream` is used in this test, it gets passed to the constructor of `ZipOutputStream`. That then means that there is no buffering involved when bytes are written out to the output stream internally by `ZipOutputStream`. The implementation of `ZipOutputStream` writes out the `ZipEntry`'s comment (if any) in one single `write(byte[])` call on the outputstream, so it's guaranteed that the `byte[] b` passed in here will actually have the entire comment (from `off` to `len`). So this `Arrays.equals(...)` check is then guaranteed to pass (when that specific entry does get written out). So this check looks good to me.

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PR Review Comment: https://git.openjdk.org/jdk/pull/12991#discussion_r1393760810