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2677.directory_entry::status is not allowed to be cached as a quality-of-implementation issue

Section: 31.12.10.4[fs.dir.entry.obs]Status:ResolvedSubmitter: Billy O'NealOpened: 2016-03-03Last modified: 2020-09-06

Priority:2

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Discussion:

To fix multi-threading problems indirectory_entry, caching behaviorwas removed from the type. This is bad for performance reasons,because the values can no longer be cached from the result ofreaddiron POSIX platforms, or fromFindFirstFile/FindNextFile on Windows.

It appears that the intent was to allow implementers to fill in thevalues fordirectory_entry::status insidedirectory_iterator, but ascurrently specified:

Returns:status(path()[, ec]).

This is not allowed to be cached, because the target of the path canchange. For example, consider the following program:

#include <stdio.h>#include <stdlib.h>#include <filesystem>#include <fstream>using namespace std;namespace fs = ::std::filesystem;void verify(const bool b, const char * const msg) {    if (!b) {        printf("fail: %s\n", msg);        exit(1);    }}void touch_file(const char * const filename) {    ofstream f(filename);    f << '\n' << endl;    verify(f.good(), "File write failed");}int main() {    fs::remove_all("subDir");    fs::create_directory("subDir");    fs::create_directory("subDir/child");    touch_file("subDir/child/child");    fs::current_path("./subDir");    fs::directory_iterator dir(".");    ++dir;    fs::directory_entry entry = *dir;    verify(entry == "./child",      "unexpected subdirectory"); //enumerating "subDir" returned the directory "child"    fs::file_status status = entry.status();    verify(status.type() == fs::file_type::directory,        "subDir/child was not a directory");    fs::current_path("./child");    status = entry.status(); // REQUIRED to re-stat() on POSIX,                             // GetFileAttributes() on Windows    verify(status.type() == fs::file_type::regular,        "subDir/child/child was not a regular file");    return 0;}

directory_entry should be re-specified to allow implementers to cache the value ofstatus(path) at the timeirectory_iterator was incremented to avoid repeatedstat() /GetFileAttributes calls. (This may mean additional constructors are necessary fordirectory_entry as well)

[2016-04, Issues Telecon]

Beman is working on a paper to address this.

[2016-08, Beman comments]

This will be resolved byP0317R1, Directory Entry Caching for Filesystem.

Fri AM: Moved to Tentatively Resolved

Proposed resolution: