[Issue 14861] Error in stdio.d in LockingTextReader.readFront()

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=14861

ag0aep6g at gmail.com changed:

           What    |Removed                     |Added
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           Keywords|                            |pull
                 CC|                            |ag0aep6g at gmail.com
           Assignee|nobody at puremagic.com        |ag0aep6g at gmail.com

--- Comment #3 from ag0aep6g at gmail.com ---
Reduced test case:

----
import std.stdio;
import std.array: replicate;

void main()
{
    File fw = File("panic.csv", "w");
    fw.rawWrite("a".replicate(16383) ~ "\xD1\x91\xD1\x82");
        /* \xD1\x91 = U+0451 CYRILLIC SMALL LETTER IO */
        /* \xD1\x82 = U+0442 CYRILLIC SMALL LETTER TE */
    fw.close();

    File fr = File("panic.csv", "r");
    fr.rawRead(new char[16383]);

    auto ltr = LockingTextReader(fr);
    assert(ltr.front == '\u0451'); /* passes */
    ltr.popFront(); /* "Invalid UTF-8 sequence" */
    assert(ltr.front == '\u0442');
    ltr.popFront();
    assert(ltr.empty);
}
----

LockingTextReader essentially does this:

----
    auto fps = fr.getFP();
    auto fp = cast(_iobuf*) fps;

    assert(FGETC(fp) == '\xD1'); /* passes */
    assert(FGETC(fp) == '\x91'); /* passes */

    assert(ungetc('\x91', fps) == '\x91'); /* passes */
    assert(ungetc('\xD1', fps) == '\xD1'); /* passes */

    assert(FGETC(fp) == '\xD1'); /* passes */
    assert(FGETC(fp) == '\x91'); /* fails */
----

The problem is that ungetc is called multiple times. Apparently, the Windows 32
C runtime doesn't like that under these specific circumstances.

Checking the documentation for ungetc, calling it more than once is actually
not guaranteed to work.

Here's a PR that replaces the ungetc calls with ftell/fseek:
https://github.com/D-Programming-Language/phobos/pull/3622

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