Change the behavior of open on files to not truncate by default

Change the behavior of open on files to not truncate by default
when opening for writing.

We have an explicit truncate method in the API and truncating
the file is a bit hostile to users who want to modify or append
to an existing file. :-)


R=whesse@google.com
BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=2941

[Bill Hesse, 2011-12-23 14:22:23]

I really prefer files to be truncated be default - that is what every Posix,
python, C, node.js programmer is used to.
Shouldn't we have mode arguments to File.open(mode) that
are File.READ, File.WRITE, and File.APPEND, instead of a boolean? I'm not sure
about the naming - it could also be Mode.read, Mode.write, and Mode.append.


Or else add a second argument to open, File.open(bool writable = false, bool
append = false).

I've been relying on File.open overwriting any existing contents of the file,
and I think most programmers will rely on that.

If we make the writable mode of File.open append, then almost every use of
File.open(writable=true) will be a bug unless the programmer also writes
truncate();.

The code looks good to me, but I don't agree with this change.

[Bill Hesse, 2012-01-02 14:29:41]

Also see issue dart:1017

[Mads Ager (google), 2012-01-03 15:18:47]

I agree with your comments Bill. Thanks. Changed to use FileMode type instead.

FileMode:WRITE truncates.
FileMode::APPEND does not.

I will update the testing binaries for all platforms when done with the review
process.

[Bill Hesse, 2012-01-03 18:28:47]

LGTM.  I guess it is good to distinguish between the internal (C++) mode
representation and the Dart FileMode representation.

http://codereview.chromium.org/9034005/diff/5001/tests/standalone/src/FileTes...
File tests/standalone/src/FileTest.dart (right):

http://codereview.chromium.org/9034005/diff/5001/tests/standalone/src/FileTes...
tests/standalone/src/FileTest.dart:660: // Test that opens the same file for
writing twice to test that
I wouldn't say "writing twice" but "for writing, then for appending"

http://codereview.chromium.org/9034005/diff/5001/tests/standalone/src/FileTes...
tests/standalone/src/FileTest.dart:661: // the file is not truncated when opened
for writing.
for appending.

[Mads Ager (google), 2012-01-04 10:27:51]

http://codereview.chromium.org/9034005/diff/5001/tests/standalone/src/FileTes...
File tests/standalone/src/FileTest.dart (right):

http://codereview.chromium.org/9034005/diff/5001/tests/standalone/src/FileTes...
tests/standalone/src/FileTest.dart:660: // Test that opens the same file for
writing twice to test that
On 2012/01/03 18:28:47, Bill Hesse wrote:
> I wouldn't say "writing twice" but "for writing, then for appending"

Done.

http://codereview.chromium.org/9034005/diff/5001/tests/standalone/src/FileTes...
tests/standalone/src/FileTest.dart:661: // the file is not truncated when opened
for writing.
On 2012/01/03 18:28:47, Bill Hesse wrote:
> for appending.

Done.