ORC-1684: [C++] Find tzdb without TZDIR when in conda-environments

### What changes were proposed in this pull request?

Find tzdb without having to set `TZDIR` when in a conda-environment (where `tzdata` [has](https://conda-metadata-app.streamlit.app/?q=conda-forge%2Fnoarch%2Ftzdata-2024a-h0c530f3_0.conda) a uniform location of `$CONDA_PREFIX/share/zoneinfo` across all platforms).

### Why are the changes needed?

This is due to issues in arrow (see apache/arrow#36026) that cannot really be fixed there, as it assumes that orc >=2.0 knows how to find the tzdb. Having to set `TZDIR` in all user environments is an intrusive change that should be avoided, and since the cost here is checking a single environment variable, it's hopefully not too onerous for consideration.

### How was this patch tested?

CI here

### Was this patch authored or co-authored using generative AI tooling?

No

CC wgtmac

See also: #1587

Closes #1882 from h-vetinari/tzdb.

Authored-by: H. Vetinari <h.vetinari@gmx.com>
Signed-off-by: Gang Wu <ustcwg@gmail.com>
(cherry picked from commit e89ca33)
Signed-off-by: Gang Wu <ustcwg@gmail.com>

c++/src/Timezone.cc

@@ -655,10 +655,18 @@ namespace orc {
     epoch = utcEpoch - getVariant(utcEpoch).gmtOffset;
   }
 
-  const char* getTimezoneDirectory() {
+  std::string getTimezoneDirectory() {
     const char* dir = getenv("TZDIR");
     if (!dir) {
-      dir = DEFAULT_TZDIR;
+      // this is present if we're in an activated conda environment
+      const char* condaPrefix = getenv("CONDA_PREFIX");
+      if (condaPrefix) {
+        std::string condaDir(condaPrefix);
+        condaDir += "/share/zoneinfo";
+        return condaDir;
+      } else {
+        dir = DEFAULT_TZDIR;
+      }
     }
     return dir;
   }

c++/test/TestTimezone.cc

@@ -21,6 +21,7 @@
 #include "wrap/gmock.h"
 #include "wrap/gtest-wrapper.h"
 
+#include <algorithm>
 #include <cstdlib>
 #include <iostream>
 #include <vector>
@@ -421,20 +422,61 @@ namespace orc {
   }
 
   TEST(TestTimezone, testMissingTZDB) {
-    const char* tzDirBackup = std::getenv("TZDIR");
-    if (tzDirBackup != nullptr) {
+    const char* tzDir = std::getenv("TZDIR");
+    std::string tzDirBackup;
+    if (tzDir != nullptr) {
+      // std::string creates a deepcopy of buffer, which avoids that
+      // unsetting environment variable wrecks pointer to tzDir
+      tzDirBackup = tzDir;
       ASSERT_TRUE(delEnv("TZDIR"));
     }
     ASSERT_TRUE(setEnv("TZDIR", "/path/to/wrong/tzdb"));
     EXPECT_THAT([]() { getTimezoneByName("America/Los_Angeles"); },
                 testing::ThrowsMessage<TimezoneError>(testing::HasSubstr(
                     "Time zone file /path/to/wrong/tzdb/America/Los_Angeles does not exist."
                     " Please install IANA time zone database and set TZDIR env.")));
-    if (tzDirBackup != nullptr) {
-      ASSERT_TRUE(setEnv("TZDIR", tzDirBackup));
+    if (!tzDirBackup.empty()) {
+      ASSERT_TRUE(setEnv("TZDIR", tzDirBackup.c_str()));
     } else {
       ASSERT_TRUE(delEnv("TZDIR"));
     }
   }
 
+  TEST(TestTimezone, testTzdbFromCondaEnv) {
+    const char* tzDir = std::getenv("TZDIR");
+    // test only makes sense if TZDIR exists
+    if (tzDir != nullptr) {
+      std::string tzDirBackup = tzDir;
+      ASSERT_TRUE(delEnv("TZDIR"));
+
+      // remove "/share/zoneinfo" from TZDIR (as set through TZDATA_DIR in CI) to
+      // get the equivalent of CONDA_PREFIX, relative to the location of the tzdb
+      std::string condaPrefix(tzDirBackup);
+      condaPrefix += "/../..";
+      ASSERT_TRUE(setEnv("CONDA_PREFIX", condaPrefix.c_str()));
+
+      // small test sample to ensure tzbd loads with CONDA_PREFIX, even without TZDIR
+      const Timezone* zrh = &getTimezoneByName("Europe/Zurich");
+      EXPECT_EQ("CET", getVariantFromZone(*zrh, "2024-03-31 00:59:59"));
+      EXPECT_EQ("CEST", getVariantFromZone(*zrh, "2024-03-31 01:00:00"));
+      EXPECT_EQ("CEST", getVariantFromZone(*zrh, "2024-10-27 00:59:59"));
+      EXPECT_EQ("CET", getVariantFromZone(*zrh, "2024-10-27 01:00:00"));
+
+      // CONDA_PREFIX contains backslashes on windows; test that this doesn't blow up
+      std::replace(condaPrefix.begin(), condaPrefix.end(), '/', '\\');
+      ASSERT_TRUE(setEnv("CONDA_PREFIX", condaPrefix.c_str()));
+
+      // as above, but different timezone to avoid hitting cache
+      const Timezone* syd = &getTimezoneByName("Australia/Sydney");
+      EXPECT_EQ("AEDT", getVariantFromZone(*syd, "2024-04-06 15:59:59"));
+      EXPECT_EQ("AEST", getVariantFromZone(*syd, "2024-04-06 16:00:00"));
+      EXPECT_EQ("AEST", getVariantFromZone(*syd, "2024-10-05 15:59:59"));
+      EXPECT_EQ("AEDT", getVariantFromZone(*syd, "2024-10-05 16:00:00"));
+
+      // restore state of environment variables
+      ASSERT_TRUE(delEnv("CONDA_PREFIX"));
+      ASSERT_TRUE(setEnv("TZDIR", tzDirBackup.c_str()));
+    }
+  }
+
 }  // namespace orc