MXS-1464 Add config 'substitute_variables'

With this variables set to true, if $VAR is used as a value in the
configuration file, then `$VAR` will be replaced with the value of
the environment variable VAR.

Documentation/Getting-Started/Configuration-Guide.md

@@ -561,6 +561,32 @@ This will log all statements that cannot be parsed completely. This may be
 useful if you suspect that MariaDB MaxScale routes statements to the wrong
 server (e.g. to a slave instead of to a master).
 
+#### `substitute_variables`
+
+Enable or disable the substitution of environment variables in the MaxScale
+configuration file. If the substitution of variables is enabled and a
+configuration line like
+```
+some_parameter=$SOME_VALUE
+```
+is encountered, then `$SOME_VALUE` will be replaced with the actual value
+of the environment variable `SOME_VALUE`. Note:
+* Variable substitution will be made _only_ if '$' is the first character
+  of the value.
+* _Everything_ following '$' is interpreted as the name of the environment
+  variable.
+* Referring to a non-existing environment variable is a fatal error.
+
+By default, the value of `substitute_variables` is `false`.
+```
+substitute_variables=true
+```
+The setting of `substitute_variables` will have an effect on all parameters
+in the all other sections, irrespective of where the `[maxscale]` section
+is placed in the configuration file. However, in the `[maxscale]` section,
+to ensure that substitution will take place, place the
+`substitute_variables=true` line first.
+
 ### REST API Configuration
 
 The MaxScale REST API is an HTTP interface that provides JSON format data

include/maxscale/config.h

@@ -157,6 +157,7 @@ extern const char CN_SSL_CERT_VERIFY_DEPTH[];
 extern const char CN_SSL_KEY[];
 extern const char CN_SSL_VERSION[];
 extern const char CN_STRIP_DB_ESC[];
+extern const char CN_SUBSTITUTE_VARIABLES[];
 extern const char CN_THREADS[];
 extern const char CN_THREAD_STACK_SIZE[];
 extern const char CN_TYPE[];
@@ -224,6 +225,7 @@ typedef struct
     char          admin_ssl_ca_cert[PATH_MAX];         /**< Admin SSL CA cert */
     int           query_retries;                       /**< Number of times a interrupted query is retried */
     time_t        query_retry_timeout;                 /**< Timeout for query retries */
+    bool          substitute_variables;                /**< Should environment variables be substituted */
 } MXS_CONFIG;
 
 /**

server/core/config.cc

@@ -135,6 +135,7 @@ const char CN_SSL_CERT_VERIFY_DEPTH[]         = "ssl_cert_verify_depth";
 const char CN_SSL_KEY[]                       = "ssl_key";
 const char CN_SSL_VERSION[]                   = "ssl_version";
 const char CN_STRIP_DB_ESC[]                  = "strip_db_esc";
+const char CN_SUBSTITUTE_VARIABLES[]          = "substitute_variables";
 const char CN_THREADS[]                       = "threads";
 const char CN_THREAD_STACK_SIZE[]             = "thread_stack_size";
 const char CN_TYPE[]                          = "type";
@@ -457,8 +458,7 @@ void fix_section_name(char *section)
  * @param value         The Parameter value
  * @return zero on error
  */
-static int
-ini_handler(void *userdata, const char *section, const char *name, const char *value)
+static int ini_handler(void *userdata, const char *section, const char *name, const char *value)
 {
     CONFIG_CONTEXT   *cntxt = (CONFIG_CONTEXT *)userdata;
     CONFIG_CONTEXT   *ptr = cntxt;

server/core/gateway.cc

@@ -2617,6 +2617,24 @@ void set_log_augmentation(const char* value)
 static int cnf_preparser(void* data, const char* section, const char* name, const char* value)
 {
     MXS_CONFIG* cnf = config_get_global_options();
+
+    if (cnf->substitute_variables)
+    {
+        if (*value == '$')
+        {
+            char* env_value = getenv(value + 1);
+
+            if (!env_value)
+            {
+                MXS_ERROR("The environment variable %s, used as value for parameter %s "
+                          "in section %s, does not exist.", value, name, section);
+                return 0;
+            }
+
+            value = env_value;
+        }
+    }
+
     char *tmp;
     /** These are read from the configuration file. These will not override
      * command line parameters but will override default values. */
@@ -2790,6 +2808,10 @@ static int cnf_preparser(void* data, const char* section, const char* name, cons
                 cnf->log_to_shm = config_truth_value((char*)value);
             }
         }
+        else if (strcmp(name, CN_SUBSTITUTE_VARIABLES) == 0)
+        {
+            cnf->substitute_variables = config_truth_value(value);
+        }
     }
 
     return 1;