Add a flat-file implementation of property_set / property_get.

[owtaylor]

For non-android systems, implement property_set() and property_get() in terms of operations on a flat file of key=value pairs. The path is hard-coded in include.mk to $sysconfdir/adb.properties.

The implementation is not particularly efficient, but designed to be safe against updates from multiple threads and processes.

[hadess]

In addition to the changes mentioned, I'd really like to have 1) room in the implementation to define some properties programmatically (quite a few of them should be provided by systemd and co.) 2) have an example file with a list of properties with a nice default value, if only to show the syntax.

[owtaylor]

Do you have an example of something that we could implement programmatically that adbd needs? It seems like most of the ro.* it reads just don't matter for the non-android case. I believe that ro.*

Hmm, an example file is a little tricky because while the file is flat:

• It doesn't support comments
• It is ordered by key value
• It gets updated by changing existing lines and inserting lines between existing lines

It's basically human readable but only human writable at an emergency. I suppose I can move to more human friendly:

• Ignore initial whitespace, and lines beginning with #
• Drop the ordering by key value - it allows future efficient bsearch lookups, but that doesn't matter
• Insert all programmatically added lines at the end of the file

then we could ship an example adb.properties with a commented out service.adb.tcp.port and a comment about syntax.

[owtaylor]

"I believe that ro.*" - meant to say that the "ro." properties are android build time, as far as I can determine - system properties aren't used for dynamic system information.

[owtaylor]

Updated my patches to:

• merge the PROP_* constants with the existing ADB_NON_ANDROID block
• make the file format human editable (commnents, no including trailing whitespace into values, etc.)
• install a sample file