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]

You already have something similar in the CFLAGS. I would prefer to keep it only there, so as to require minimal changes to the original Android code.

[owtaylor]

PROPERTY_NAME/VALUE_MAX have to be the same when properties.h is included as when the code is compiled; this is very different from PROPERTIES_PATH which only needs to be defined when compiling properties_nonandroid.c.

It would be possible, I suppose, to adjust the makefiles so that OPT_CFLAGS/OPT_CXXFLAGS are replaced with BASE_CFLAGS/BASE_CXXFLAGS and we add -D flags there, but I think that's really ugly.

What I replaced this with:

#if !ADB_NON_ANDROID
#include <sys/system_properties.h>
#else
+#define PROP_KEY_MAX 32
+#define PROP_VALUE_MAX 92
#endif

to keep this clearly in an existing non-android block?

[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