archive/tar: TestSparseFiles failing on plan9/arm builder

[0intro]

CL 60871 added TestSparseFiles. This test is failing on the plan9/arm builder.

--- FAIL: TestSparseFiles (117.51s)
    --- FAIL: TestSparseFiles/EmptyFile (0.17s)
    	tar_test.go:844: unexpected ReadFrom error: tar: write too long
    --- FAIL: TestSparseFiles/BigHole (0.25s)
    	tar_test.go:844: unexpected ReadFrom error: tar: write too long
    --- FAIL: TestSparseFiles/DataFront (0.44s)
    	tar_test.go:844: unexpected ReadFrom error: tar: write too long
    --- FAIL: TestSparseFiles/HoleFront (1.43s)
    	tar_test.go:872: sparse files mismatch
    --- FAIL: TestSparseFiles/DataMiddle (0.39s)
    	tar_test.go:844: unexpected ReadFrom error: tar: write too long
    --- FAIL: TestSparseFiles/HoleMiddle (1.41s)
    	tar_test.go:872: sparse files mismatch
    --- FAIL: TestSparseFiles/Multiple (109.91s)
    	tar_test.go:872: sparse files mismatch
FAIL
FAIL	archive/tar	140.157s

See https://build.golang.org/log/6c62187355a860390505342ac82661184645f959

[0intro]

The test is only failing on the plan9/arm builder because this builder use the Fossil file system for /tmp instead of ramfs.

It seems there is an issue with the handling of sparse files in the Fossil file system.

[gopherbot]

Change https://golang.org/cl/65352 mentions this issue: archive/tar: skip TestSparseFiles on Plan 9

[dsnet]

I'll investigate why this is failing on Plan9. On OS without sparse file support, this test should still pass. It does however rely on being able to Seek beyond the EOF and write some data, with the assumption that the data in-between is filled with zeros (and on OS+FS that support it, represented by a sparse hole).

[millerresearch]

TestSparseFiles was removed in 2017 by CL 78030.

[bcmills]

It does however rely on being able to Seek beyond the EOF and write some data, with the assumption that the data in-between is filled with zeros

That assumption does not hold for io.Seeker implementations in general anyway (see #48316). I am not sure whether it ought to always hold for os.File in particular.