DOC: configuration: update "Environment variables" chapter

There are some variables, which are set by HAProxy process (HAPROXY_*). Some
of them are handy to check or to redefine in the configuration, in order to
create conditional blocks and make the configuration more flexible. But it
wasn't clear in the documentation, which variables are really safe and usefull
to redefine and which ones could be only read via "show env" output.

Latest changes in master-worker architecture makes the existed description even
more confusing.

So let's sort all HAPROXY_* variables to four categories and let's also mark
explicitly, which ones are set in which process, when haproxy is started in
master-worker mode.

In addition, update examples in chapter "2.4. Conditional blocks". This might
bring more ideas for users how HAPROXY_* variables could be used in the
conditional blocks.

Author: einval22

doc/configuration.txt

@@ -935,7 +935,77 @@ existing variables, not empty ones.
       user "$HAPROXY_USER"
 
 Some variables are defined by HAProxy, they can be used in the configuration
-file, or could be inherited by a program (See 3.7. Programs):
+file, or could be inherited by a program (See 3.7. Programs). These variables
+are listed in the matrix below, and they are classified among four categories:
+
+  * usable: the variable is accessible from the configuration, either to be
+    resolved as-is, or used within conditional blocks or predicates to enable
+    or disable this some configuration fragments, as described in section 2.4
+    "Conditional blocks".
+
+  * modifiable: the variable can be redefined or unset in the configuration via
+    "setenv"/"unsetenv" keywords.
+
+  * listed: the variable is listed in CLI's "show env" command output,
+    described in section 9.3 "Unix Sockets commands" of the management guide.
+
+  * exported: variable is exported to launch programs in a modified environment
+    (See section 3.7 "Programs"). Note that this does not apply to external
+    checks which have their own rules regarding exported variables.
+
+There also two subcategories "master" and "worker", respectively marked 'M' and
+'W' in the table below, showing the differences between the two processes when
+HAProxy is launched in master-worker mode.
+
+  * master: the variable is set and accessible from the master process. So, it
+    will appear in the master CLI's "show env" output and it can be used in
+    conditional blocks or directives to enable some special settings for the
+    master (see examples in section 2.4 "Conditional blocks").
+
+  * worker: the variable is set and accessible from the worker process. It will
+    appear in the worker CLI's "show env" (or the master CLI's "@1 show env")
+    and it may as well condition some worker process parameters (see examples
+    from section 2.4 "Conditional blocks").
+
+In standalone mode (without "-W" option nor the "master-worker" keyword) the
+process behaves like a worker, except for variables "HAPROXY_MASTER_CLI" and
+"HAPROXY_MWORKER" which are not defined.
+
+Some variables are marked as not usable and not modifiable:
+
+  * HAPROXY_CFGFILES
+  * HAPROXY_MWORKER
+  * HAPROXY_CLI
+  * HAPROXY_MASTER_CLI
+  * HAPROXY_LOCALPEER
+
+Their values are undefined during configuration parsing, they are set later
+during the initialization. So, it's recommended not to use these variables
+within conditional blocks and not to reference them in the global section's
+"setenv"/"resetenv"/"unsetenv" keywords.
+
+The table below summaries the status of each variable for the different working
+modes:
+
+  +--------------------------+----------+---------+------------+-----------+
+  |        variable          | exported | usable  | modifiable |  listed   |
+  |                          |          +---------+------------+-----------+
+  |                          |          |  M | W  |   M  |  W  |  M  |  W  |
+  +--------------------------+----------+----+----+------+-----+-----+-----+
+  | HAPROXY_STARTUP_VERSION  |    X     |  X | X  |      |     |  X  |  X  |
+  | HAPROXY_BRANCH           |    X     |  X | X  |      |     |  X  |  X  |
+  | HAPROXY_CFGFILES         |    X     |    |    |      |     |  X  |  X  |
+  | HAPROXY_MWORKER          |    X     |    |    |      |     |  X  |  X  |
+  | HAPROXY_CLI              |          |    |    |      |     |     |  X  |
+  | HAPROXY_MASTER_CLI       |          |    |    |      |     |  X  |     |
+  | HAPROXY_LOCALPEER        |          |    |    |      |     |     |  X  |
+  | HAPROXY_HTTP_LOG_FMT     |          |    | X  |      |  X  |     |     |
+  | HAPROXY_HTTP_CLF_LOG_FMT |          |    | X  |      |  X  |     |     |
+  | HAPROXY_HTTPS_LOG_FMT    |          |    | X  |      |  X  |     |     |
+  | HAPROXY_TCP_LOG_FMT      |          |    | X  |      |  X  |     |     |
+  +--------------------------+----------+----+----+------+-----+-----+-----+
+
+The variables in question are the following:
 
 * HAPROXY_LOCALPEER: defined at the startup of the process which contains the
   name of the local peer. (See "-L" in the management guide.)
@@ -968,8 +1038,8 @@ file, or could be inherited by a program (See 3.7. Programs):
 
 * HAPROXY_MWORKER: In master-worker mode, this variable is set to 1.
 
-* HAPROXY_CLI: configured listeners addresses of the stats socket for every
-  processes, separated by semicolons.
+* HAPROXY_CLI: configured listeners addresses of the stats socket of every
+  processe, these addresses are separated by semicolons.
 
 * HAPROXY_MASTER_CLI: In master-worker mode, listeners addresses of the master
   CLI, separated by semicolons.
@@ -1006,8 +1076,6 @@ This way it is possible to emit information to help locate a rule in variables,
 logs, error statuses, health checks, header values, or even to use line numbers
 to name some config objects like servers for example.
 
-See also "external-check command" for other variables.
-
 
 2.4. Conditional blocks
 -----------------------
@@ -1139,12 +1207,41 @@ The list of currently supported predicates is the following:
 
 Example:
 
+   # 1. HAPROXY_MWORKER variable is set automatically by HAProxy in master and
+   # in worker process environments (see HAProxy variables matrix from
+   # 2.3. Environment variables). Its presence enables an additional listener.
+
+   global
+     master-worker
+
    .if defined(HAPROXY_MWORKER)
        listen mwcli_px
           bind :1111
           ...
    .endif
 
+   # 2. HAPROXY_BRANCH is set automatically by HAProxy in master and in worker
+   # process environments (see HAProxy variables matrix from 2.3. Environment
+   # variables). We check HAPROXY_BRANCH value and conditionally enable
+   # mworker-max-reloads parameter.
+
+   global
+     master-worker
+
+   .if streq("$HAPROXY_BRANCH",3.1)
+        mworker-max-reloads 5
+   .endif
+
+   # 3. Some arbitrary environment variables are set by user in the global
+   # section. If HAProxy is started in master-worker mode, they are presented in
+   # master and in worker process environments. We check values of these
+   # variables and conditionally enable ports 80 and 443. Environment variables
+   # checks can be mixed with features and version checks.
+
+   global
+     setenv WITH_SSL yes
+     unsetenv SSL_ONLY
+
    .if strneq("$SSL_ONLY",yes)
           bind :80
    .endif
@@ -11291,8 +11388,6 @@ external-check command <command>
     PATH                    The PATH environment variable used when executing
                             the command may be set using "external-check path".
 
-  See also "2.3. Environment variables" for other variables.
-
   If the command executed and exits with a zero status then the check is
   considered to have passed, otherwise the check is considered to have
   failed.