[util] Implement TryGetEnv and UnsetEnv functions

The TryGetEnv function can be used to distinguish the case when a variable is not defined from the case when it has an empty value.
4aab7a2a39f4b9107b0b1d1ef639dc2c41c04f42

1 files changed, 32 insertions, 2 deletions

diff --git a/util/system/env.h b/util/system/env.h
index e2ccdd1e95..55f43bd9b8 100644
--- a/util/system/env.h
+++ b/util/system/env.h
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <util/generic/fwd.h>
 #include <util/generic/string.h>
 
 /**
@@ -19,10 +20,27 @@
 TString GetEnv(const TString& key, const TString& def = TString());
 
 /**
+ * Search the environment list provided by the host environment for associated variable.
+ *
+ * @param key               String identifying the name of the environmental variable to look for
+ *
+ * @return                  String that is associated with the matched environment
+ *                          variable or empty optional value if such variable is missing.
+ *
+ * @throws TSystemError     If name of the variable has invalid format
+ *
+ * @note                    Use it only in pair with `SetEnv` as there may be inconsistency
+ *                          in their behaviour otherwise.
+ * @note                    Calls to `TryGetEnv` and `SetEnv` from different threads must be synchronized.
+ * @see                     SetEnv
+ */
+TMaybe<TString> TryGetEnv(const TString& key);
+
+/**
  * Add or change environment variable provided by the host environment.
  *
- * @key         String identifying the name of the environment variable to set or change
- * @value       Value to assign
+ * @param key   String identifying the name of the environment variable to set or change
+ * @param value Value to assign
 
  * @note        Use it only in pair with `GetEnv` as there may be inconsistency in their behaviour
  *              otherwise.
@@ -30,3 +48,15 @@ TString GetEnv(const TString& key, const TString& def = TString());
  * @see         GetEnv
  */
 void SetEnv(const TString& key, const TString& value);
+
+/**
+ * Remove environment variable from the host environment.
+ *
+ * @param key   String identifying the name of the environment variable to remove
+ *
+ * @note        If key does not exist in the environment, then the environment is unchanged,
+ *              and the function returns normally.
+ * @note        Calls to `GetEnv` and `SetEnv` from different threads must be synchronized.
+ * @see         GetEnv
+ */
+void UnsetEnv(const TString& key);