Optimize FilePath.readToString

[jglick]

While discussing an OutOfMemoryError whose investigation led to jenkinsci/workflow-durable-task-step-plugin#96, @dwnusbaum pointed out that FilePath.readToString winds up allocating a IOUtils.DEFAULT_BUFFER_SIZE ~ 4Kib buffer. This is clearly inefficient for the common case of reading a small text file, not to mention the overhead of using ProxyOutputStream in the Remoting channel if the file is remote. Better in typical cases to just read the whole file at once remotely and send back one packet with the result. (The current implementation would only make sense for huge text files, and using this method for such cases is likely a sign of poor code anyway.)

Note that I avoided FileUtils.readFileToString from Commons IO since it has the same 4Kib buffer size, perversely.

Also note that the rewritten implementation alters the behavior slightly: the default charset being used is now that of the agent, rather than the master. This is generally what you would want, and matches the behavior of FilePath.write(content, null).

Proposed changelog entries

• Internal optimization in the method to read a remote file as text.

[oleg-nenashev]

👍