Don't clobber a previously set level filter

Cargo.toml

@@ -33,5 +33,9 @@ harness = false
 name = "log-in-log"
 harness = false
 
+[[test]]
+name = "init-twice-retains-filter"
+harness = false
+
 [features]
 default = ["termcolor", "atty", "humantime", "regex"]

src/lib.rs

@@ -646,8 +646,14 @@ impl Builder {
     pub fn try_init(&mut self) -> Result<(), SetLoggerError> {
         let logger = self.build();
 
-        log::set_max_level(logger.filter());
-        log::set_boxed_logger(Box::new(logger))
+        let max_level = logger.filter();
+        let r = log::set_boxed_logger(Box::new(logger));
+
+        if r.is_ok() {
+            log::set_max_level(max_level);
+        }
+
+        r
     }
 
     /// Initializes the global logger with the built env logger.

tests/init-twice-retains-filter.rs

@@ -0,0 +1,40 @@
+extern crate log;
+extern crate env_logger;
+
+use std::process;
+use std::env;
+use std::str;
+
+fn main() {
+    if env::var("YOU_ARE_TESTING_NOW").is_ok() {
+        // Init from the env (which should set the max level to `Debug`)
+        env_logger::init();
+
+        assert_eq!(log::LevelFilter::Debug, log::max_level());
+
+        // Init again using a different max level
+        // This shouldn't clobber the level that was previously set
+        env_logger::Builder::new()
+            .parse("info")
+            .try_init()
+            .unwrap_err();
+
+        assert_eq!(log::LevelFilter::Debug, log::max_level());
+        return
+    }
+
+    let exe = env::current_exe().unwrap();
+    let out = process::Command::new(exe)
+        .env("YOU_ARE_TESTING_NOW", "1")
+        .env("RUST_LOG", "debug")
+        .output()
+        .unwrap_or_else(|e| panic!("Unable to start child process: {}", e));
+    if out.status.success() {
+        return
+    }
+
+    println!("test failed: {}", out.status);
+    println!("--- stdout\n{}", str::from_utf8(&out.stdout).unwrap());
+    println!("--- stderr\n{}", str::from_utf8(&out.stderr).unwrap());
+    process::exit(1);
+}