From c032446c3fe39bc512af0e91c41561db334ff056 Mon Sep 17 00:00:00 2001
From: Dan Gohman <sunfish@mozilla.com>
Date: Mon, 9 Mar 2020 15:57:06 -0700
Subject: [PATCH] Log to stderr by default.

Change the default from file-per-thread-logger to pretty-env-logger,
which is more common in Rust projects, and change the option from `-d`
to `--log-to-files`.
---
 docs/cli-logging.md      | 26 ++++++++++++++++++++++++++
 docs/cli.md              |  4 ++--
 src/commands/run.rs      |  6 +++---
 src/commands/wasm2obj.rs |  6 +++---
 src/commands/wast.rs     |  6 +++---
 src/lib.rs               |  6 +++---
 6 files changed, 40 insertions(+), 14 deletions(-)
 create mode 100644 docs/cli-logging.md

diff --git a/docs/cli-logging.md b/docs/cli-logging.md
new file mode 100644
index 000000000000..333825328bea
--- /dev/null
+++ b/docs/cli-logging.md
@@ -0,0 +1,26 @@
+# Logging in the `wasmtime` CLI
+
+Wasmtime's libraries use Rust's [`log`] crate to log diagnostic
+information, and the `wasmtime` CLI executable uses [`pretty_env_logger`]
+by default for logging this information to the console.
+
+Basic logging is controlled by the `RUST_LOG` environment variable. For example,
+To enable logging of WASI system calls, similar to the `strace` command on Linux,
+set `RUST_LOG=wasi_common=trace`.
+
+```sh
+$ RUST_LOG=wasi_common=trace wasmtime hello.wasm
+[...]
+ TRACE wasi_common::hostcalls_impl::fs                       > fd_write(fd=1, iovs_ptr=0x10408, iovs_len=1, nwritten=0x10404)
+Hello, world!
+ TRACE wasi_common::hostcalls_impl::fs                       >      | *nwritten=14
+ TRACE wasi_common::hostcalls                                >      | errno=ESUCCESS (No error occurred. System call completed successfully.)
+ TRACE wasi_common::hostcalls_impl::misc                     > proc_exit(rval=1)
+```
+
+Wasmtime can also redirect the log messages into log files, with the
+`--log-to-files` option. It creates one file per thread within Wasmtime, with
+the files named `wasmtime.dbg.*`.
+
+[`log`]: https://crates.io/crates/log
+[`pretty_env_logger`]: https://crates.io/crates/pretty_env_logger
diff --git a/docs/cli.md b/docs/cli.md
index 3f861e1143f9..3dd408624747 100644
--- a/docs/cli.md
+++ b/docs/cli.md
@@ -20,5 +20,5 @@ $ wasmtime --invoke _start foo.wasm
 ```
 
 For more information be sure to check out [how to install the
-CLI](cli-install.md) as well as [the list of options you can
-pass](cli-options.md).
+CLI](cli-install.md), [the list of options you can
+pass](cli-options.md), and [how to enable logging](cli-logging.md).
diff --git a/src/commands/run.rs b/src/commands/run.rs
index d5fa7ee0a886..7407f83c49f1 100644
--- a/src/commands/run.rs
+++ b/src/commands/run.rs
@@ -89,11 +89,11 @@ pub struct RunCommand {
 impl RunCommand {
     /// Executes the command.
     pub fn execute(&self) -> Result<()> {
-        if self.common.debug {
-            pretty_env_logger::init();
-        } else {
+        if self.common.log_to_files {
             let prefix = "wasmtime.dbg.";
             init_file_per_thread_logger(prefix);
+        } else {
+            pretty_env_logger::init();
         }
 
         let config = self.common.config()?;
diff --git a/src/commands/wasm2obj.rs b/src/commands/wasm2obj.rs
index 494857b2a2dc..dbfd89042bdd 100644
--- a/src/commands/wasm2obj.rs
+++ b/src/commands/wasm2obj.rs
@@ -54,11 +54,11 @@ impl WasmToObjCommand {
     }
 
     fn handle_module(&self) -> Result<()> {
-        if self.common.debug {
-            pretty_env_logger::init();
-        } else {
+        if self.common.log_to_files {
             let prefix = "wasm2obj.dbg.";
             init_file_per_thread_logger(prefix);
+        } else {
+            pretty_env_logger::init();
         }
 
         let cache_config = if self.common.disable_cache {
diff --git a/src/commands/wast.rs b/src/commands/wast.rs
index 4ac459319aea..d09fba9841d7 100644
--- a/src/commands/wast.rs
+++ b/src/commands/wast.rs
@@ -26,11 +26,11 @@ pub struct WastCommand {
 impl WastCommand {
     /// Executes the command.
     pub fn execute(&self) -> Result<()> {
-        if self.common.debug {
-            pretty_env_logger::init();
-        } else {
+        if self.common.log_to_files {
             let prefix = "wast.dbg.";
             init_file_per_thread_logger(prefix);
+        } else {
+            pretty_env_logger::init();
         }
 
         let config = self.common.config()?;
diff --git a/src/lib.rs b/src/lib.rs
index dc9b1bfca2ff..89c5f417a526 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -88,9 +88,9 @@ struct CommonOptions {
     #[structopt(long, conflicts_with = "lightbeam")]
     cranelift: bool,
 
-    /// Enable debug output
-    #[structopt(short, long)]
-    debug: bool,
+    /// Log to per-thread log files instead of stderr.
+    #[structopt(long)]
+    log_to_files: bool,
 
     /// Generate debug information
     #[structopt(short = "g")]