Fix #11000: getInstalledTZNames can't handle a trailing slash.

Normally, on most systems, PosixTimeZone.getInstalledTZNames uses
/usr/share/zoneinfo/ for the time zone database directory, but it can be
given a different directory, and it will use what TZDIR has been set to
if it's been set (since the function used as the default argument checks
TZDIR).

So, while the bug refers specifically to TZDIR, that's just one way that
the alternate time zone database directory can be passed to
getInstalledTZNames.

If getInstalledTZNames is given a directory name which does not end with
a slash, then the logic for stripping off the time zone database
directory name from the full paths to each time zone file leaves a slash
on the front of the time zone name, which then doesn't work correctly,
since it's not supposed to start with a slash.

So, this fixes it so that the time zone names don't end up with slashes
due to how many slashes the time zone database directory name does or
doesn't have.

I tested the subName argument as part of this in case it mattered and
was somewhat horrified to realize that it had not actually been tested
by any of the existing tests. So, I guess that I screwed up on that
count. Fortunately, it seems to work correctly.

Author: jmdavis

std/datetime/timezone.d

@@ -2474,6 +2474,10 @@ public:
                     {
                         auto tzName = de.name[tzDatabaseDir.length .. $];
 
+                        // For the case where tzDatabaseDir does not have a trailing slash.
+                        if (tzName.length > 1 && tzName[0] == '/')
+                            tzName = tzName[1 .. $];
+
                         if (!tzName.extension().empty ||
                             !tzName.startsWith(subName) ||
                             baseName(tzName) == "leapseconds" ||
@@ -2537,6 +2541,119 @@ public:
         }
     }
 
+    // https://github.com/dlang/phobos/issues/11000
+    version (Posix) @safe unittest
+    {
+        version (Android)
+        {}
+        else
+        {
+            import std.algorithm.searching : canFind;
+            import std.file : chdir, copy, exists, getcwd, mkdirRecurse, rmdirRecurse, tempDir;
+            import std.path : buildPath;
+
+            immutable baseDir = buildPath(tempDir, "tztest");
+            immutable tzDir = buildPath(baseDir, "tz");
+            immutable tzDirSlash = buildPath(baseDir, "tz/");
+            immutable tzDirDoubleSlash = buildPath(baseDir, "tz//");
+            assert(tzDirSlash[$ - 1] == '/'); // just in case buildPath ever strips the slash
+
+            scope(failure) if (baseDir.exists) rmdirRecurse(baseDir);
+
+            mkdirRecurse(buildPath(tzDir, "America"));
+            mkdirRecurse(buildPath(tzDir, "Europe"));
+
+            copy(buildPath(defaultTZDatabaseDir, "America/Denver"),
+                 buildPath(tzDir, "America/Denver"));
+            copy(buildPath(defaultTZDatabaseDir, "America/Denver"),
+                 buildPath(tzDir, "America/Chicago"));
+            copy(buildPath(defaultTZDatabaseDir, "Europe/London"),
+                 buildPath(tzDir, "Europe/London"));
+            copy(buildPath(defaultTZDatabaseDir, "UTC"),
+                 buildPath(tzDir, "UTC"));
+
+            foreach (dir; [tzDir, tzDirSlash, tzDirDoubleSlash])
+            {
+                {
+                    auto names = getInstalledTZNames("", dir);
+                    assert(names.length == 4);
+                    assert(names.canFind("America/Denver"));
+                    assert(names.canFind("America/Chicago"));
+                    assert(names.canFind("Europe/London"));
+                    assert(names.canFind("UTC"));
+                }
+                {
+                    auto names = getInstalledTZNames("America", dir);
+                    assert(names.length == 2);
+                    assert(names.canFind("America/Denver"));
+                    assert(names.canFind("America/Chicago"));
+                }
+            }
+
+            immutable cwd = getcwd();
+            scope(exit) chdir(cwd);
+
+            chdir(baseDir);
+            foreach (dir; ["tz", "tz/", "tz///"])
+            {
+                {
+                    auto names = getInstalledTZNames("", dir);
+                    assert(names.length == 4);
+                    assert(names.canFind("America/Denver"));
+                    assert(names.canFind("America/Chicago"));
+                    assert(names.canFind("Europe/London"));
+                    assert(names.canFind("UTC"));
+                }
+                {
+                    auto names = getInstalledTZNames("America", dir);
+                    assert(names.length == 2);
+                    assert(names.canFind("America/Denver"));
+                    assert(names.canFind("America/Chicago"));
+                }
+            }
+
+            immutable other = buildPath(baseDir, "other");
+            mkdirRecurse(other);
+            chdir(other);
+            foreach (dir; ["../tz", "../tz/", "..///tz/////"])
+            {
+                {
+                    auto names = getInstalledTZNames("", dir);
+                    assert(names.length == 4);
+                    assert(names.canFind("America/Denver"));
+                    assert(names.canFind("America/Chicago"));
+                    assert(names.canFind("Europe/London"));
+                    assert(names.canFind("UTC"));
+                }
+                {
+                    auto names = getInstalledTZNames("America", dir);
+                    assert(names.length == 2);
+                    assert(names.canFind("America/Denver"));
+                    assert(names.canFind("America/Chicago"));
+                }
+            }
+
+            chdir(tzDir);
+            foreach (dir; [".", "./", ".///"])
+            {
+                {
+                    auto names = getInstalledTZNames("", dir);
+                    assert(names.length == 4);
+                    assert(names.canFind("America/Denver"));
+                    assert(names.canFind("America/Chicago"));
+                    assert(names.canFind("Europe/London"));
+                    assert(names.canFind("UTC"));
+                }
+                {
+                    auto names = getInstalledTZNames("America", dir);
+                    assert(names.length == 2);
+                    assert(names.canFind("America/Denver"));
+                    assert(names.canFind("America/Chicago"));
+                }
+            }
+        }
+    }
+
 
 private: