Better error & download url if ffmpeg is missing (#7991)

### What

* part of #7607 

<img width="1012" alt="image"
src="https://github.com/user-attachments/assets/317d2d07-d544-4743-86ef-c1ae51168b16">


Future work:
* download for you in the viewer to a location we manage
* have location be an application setting

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7991?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7991?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!
* [x] If have noted any breaking changes to the log API in
`CHANGELOG.md` and the migration guide

- [PR Build Summary](https://build.rerun.io/pr/7991)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

crates/store/re_video/src/decode/ffmpeg.rs

@@ -19,7 +19,15 @@ use super::{AsyncDecoder, Chunk, Frame, OutputCallback};
 
 #[derive(thiserror::Error, Debug)]
 pub enum Error {
-    #[error("Failed to start ffmppeg: {0}")]
+    #[error("Couldn't find an installation of the FFmpeg executable.")]
+    FfmpegNotInstalled {
+        /// Download URL for the latest version of `FFmpeg` on the current platform.
+        /// None if the platform is not supported.
+        // TODO(andreas): as of writing, ffmpeg-sidecar doesn't define a download URL for linux arm.
+        download_url: Option<&'static str>,
+    },
+
+    #[error("Failed to start FFmpeg: {0}")]
     FailedToStartFfmpeg(std::io::Error),
 
     #[error("Failed to get stdin handle")]
@@ -31,34 +39,38 @@ pub enum Error {
     #[error("No frame info received, this is a likely a bug in Rerun")]
     NoFrameInfo,
 
-    #[error("Failed to write data to ffmpeg: {0}")]
+    #[error("Failed to write data to FFmpeg: {0}")]
     FailedToWriteToFfmpeg(std::io::Error),
 
     #[error("Bad video data: {0}")]
     BadVideoData(String),
 
-    #[error("FFMPEG error: {0}")]
+    #[error("FFmpeg error: {0}")]
     Ffmpeg(String),
 
-    #[error("FFMPEG fatal error: {0}")]
+    #[error("FFmpeg fatal error: {0}")]
     FfmpegFatal(String),
 
-    #[error("FFMPEG IPC error: {0}")]
+    #[error("FFmpeg IPC error: {0}")]
     FfmpegSidecar(String),
 
-    #[error("FFMPEG exited unexpectedly with code {0:?}")]
+    #[error("FFmpeg exited unexpectedly with code {0:?}")]
     FfmpegUnexpectedExit(Option<std::process::ExitStatus>),
 
-    #[error("FFMPEG output a non-image chunk when we expected only images.")]
+    #[error("FFmpeg output a non-image chunk when we expected only images.")]
     UnexpectedFfmpegOutputChunk,
 
-    #[error("Failed to send video frame info to the ffmpeg read thread.")]
+    #[error("Failed to send video frame info to the FFmpeg read thread.")]
     BrokenFrameInfoChannel,
 }
 
 impl From<Error> for super::Error {
     fn from(err: Error) -> Self {
-        Self::Ffmpeg(std::sync::Arc::new(err))
+        if let Error::FfmpegNotInstalled { download_url } = err {
+            Self::FfmpegNotInstalled { download_url }
+        } else {
+            Self::Ffmpeg(std::sync::Arc::new(err))
+        }
     }
 }
 
@@ -99,6 +111,12 @@ impl FfmpegProcessAndListener {
     ) -> Result<Self, Error> {
         re_tracing::profile_function!();
 
+        if !ffmpeg_sidecar::command::ffmpeg_is_installed() {
+            return Err(Error::FfmpegNotInstalled {
+                download_url: ffmpeg_sidecar::download::ffmpeg_download_url().ok(),
+            });
+        }
+
         let mut ffmpeg = FfmpegCommand::new()
             .hide_banner()
             // "Reduce the latency introduced by buffering during initial input streams analysis."
@@ -303,7 +321,7 @@ fn read_ffmpeg_output(
             FfmpegEvent::Log(LogLevel::Unknown, msg) => {
                 if msg.contains("system signals, hard exiting") {
                     // That was probably us, killing the process.
-                    re_log::debug!("ffmpeg process for {debug_name} was killed");
+                    re_log::debug!("FFmpeg process for {debug_name} was killed");
                     return;
                 }
                 if !should_ignore_log_msg(&msg) {
@@ -462,12 +480,12 @@ fn read_ffmpeg_output(
             }
 
             FfmpegEvent::ParsedVersion(ffmpeg_version) => {
-                re_log::debug_once!("ffmpeg version is: {}", ffmpeg_version.version);
+                re_log::debug_once!("FFmpeg version is: {}", ffmpeg_version.version);
             }
 
             FfmpegEvent::ParsedConfiguration(ffmpeg_configuration) => {
                 re_log::debug_once!(
-                    "ffmpeg configuration: {:?}",
+                    "FFmpeg configuration: {:?}",
                     ffmpeg_configuration.configuration
                 );
             }

crates/store/re_video/src/decode/mod.rs

@@ -81,8 +81,10 @@
 mod async_decoder_wrapper;
 #[cfg(with_dav1d)]
 mod av1;
+
 #[cfg(with_ffmpeg)]
 mod ffmpeg;
+
 #[cfg(target_arch = "wasm32")]
 mod webcodecs;
 
@@ -117,6 +119,15 @@ pub enum Error {
     #[error(transparent)]
     Ffmpeg(std::sync::Arc<ffmpeg::Error>),
 
+    // We need to check for this one and don't want to infect more crates with the feature requirement.
+    #[error("Couldn't find an installation of the FFmpeg executable.")]
+    FfmpegNotInstalled {
+        /// Download URL for the latest version of `FFmpeg` on the current platform.
+        /// None if the platform is not supported.
+        // TODO(andreas): as of writing, ffmpeg-sidecar doesn't define a download URL for linux arm.
+        download_url: Option<&'static str>,
+    },
+
     #[error("Unsupported bits per component: {0}")]
     BadBitsPerComponent(usize),
 }

crates/viewer/re_data_ui/src/video.rs

@@ -154,6 +154,15 @@ pub fn show_video_blob_info(
 
                         Err(err) => {
                             ui.error_label_long(&err.to_string());
+
+                            if let re_renderer::video::VideoPlayerError::Decoding(
+                                re_video::decode::Error::FfmpegNotInstalled {
+                                    download_url: Some(url),
+                                },
+                            ) = err
+                            {
+                                ui.markdown_ui(&format!("You can download a build of `FFmpeg` [here]({url}). For Rerun to be able to use it, its binaries need to be reachable from `PATH`."));
+                            }
                         }
                     }
                 }