pipe_channel not reading all data in a message

[nabilschear]

When reading data from a table using generateInNamespace, the pipe channel implementation would only try one read syscall to read the message from the osquery process running in the namespace. Under certain circumstances, this single read call would be insufficient to read all the data in the message and the query would fail.

Example:

# osqueryi --json "SELECT source_table.* FROM deb_packages AS source_table JOIN docker_containers AS dc ON source_table.pid_with_namespace = dc.pid WHERE source_table.pid_with_namespace in (select pid from docker_containers);"
E0604 23:00:14.790307 256479 linux_table_container_ipc.cpp:446] Table deb_packages failed to retrieve QueryData from the container: Failed to read the entire message from the pipe of table deb_packages, read only 64436/113588 bytes

This PR fixes this problem by putting the read syscall in a loop that continues to read until all data has been returned.

cc @Smjert related to this thread: https://osquery.slack.com/archives/C08V7KTJB/p1621533600027800

[Smjert]

Thank you for the PR!

I seem to have forgotten that a read can return less bytes than requested if only some are immediately available in the pipe, even if blocking ^^'.

fixed