FileBasedSink could write tmp files above the output prefix.

FileBasedWriteOperation.TemporaryDirectoryBuilder tempDirBuilder =
new FileBasedWriteOperation.TemporaryDirectoryBuilder();
tempDirBuilder.apply("/home/output/"),

apply() returns "/home/temp-beam-output-2017-01-25_22-14-49"

And, it was not under the output prefix "/home/output/".

Examples of Path resolve:
Paths.get("/home/output").resolve("temp-beam");
Paths.get("/home/output/").resolve("temp-beam");
/home/output/temp-beam
/home/output/temp-beam

Paths.get("/home/output").resolve("..").resolve("temp-beam");
Paths.get("/home/output/").resolve("..").resolve("temp-beam");
/home/output/../temp-beam
/home/output/../temp-beam

Paths.get("/home/output").resolveSibling("temp-beam");
Paths.get("/home/output/").resolveSibling("temp-beam");
/home/temp-beam
/home/temp-beam

Imported from Jira [BEAM-1315](https://issues.apache.org/jira/browse/BEAM-1315). Original Jira may contain additional context.
Reported by: peihe0@gmail.com.

Start by locating FileBasedWriteOperation.TemporaryDirectoryBuilder and inspect how apply() constructs its path. Reproduce the example with /home/output/ and verify that the temporary directory remains under that output prefix. Done means the reported path behavior is corrected and the relevant behavior is covered by a regression test.

Written by the indexing model from the issue text.