ipx-directory: the directory of public podcasts ipx servers share (#204)

A new crate and service, crates/directory, live at ipx-dir.sdf1.net. ipx
servers send the public feeds their people subscribe to (POST /v1/snapshot,
replacing what that token sent before); the directory fetches each feed's
title, artwork, category, description and latest twenty items every six
hours with ipx-feed, and lists them A to Z and by subscribers summed over
every server. It holds metadata only, never a file or a file's address.

Tokens are in its config file as SHA-256 hashes, each ro, rw or rwa, and
the file is read again within a minute of a change, so adding and revoking
one is an edit and nothing else; a revoked token takes its server's counts.
At first they were to come from environment variables, which would have
meant recreating the container to add one. A file that does not load keeps
the tokens as they were.

Every address sent is checked with ipx_feed::url_looks_private, the check
ipx makes before sending, so one server's miss does not publish a paid feed
to every other; an rwa token can remove and block what both miss.

Deployed: ipx-directory in the ipx Arcane project, database ipx_directory
(its own login) on the databases project's Postgres, which db-backup
already covers, and a tunnel route with no Access in front. The store's
tests run on Postgres as well as SQLite when IPX_TEST_DATABASE_URL is set.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/directory/src/config.rs

@@ -0,0 +1,172 @@
+//! The directory's config file: where it listens, where its database is, and the tokens it
+//! accepts. Tokens are only ever changed here, never over the API, so whoever can edit this
+//! file decides who gets in (#204).
+
+use anyhow::{Context, Result, bail};
+use serde::Deserialize;
+use std::path::Path;
+
+#[derive(Debug, Clone, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct Config {
+    #[serde(default = "default_bind")]
+    pub bind: String,
+    /// The environment variable holding the database URL, so the file holds no secret.
+    #[serde(default = "default_database_url_env")]
+    pub database_url_env: String,
+    #[serde(default)]
+    pub tokens: Vec<TokenCfg>,
+}
+
+fn default_bind() -> String {
+    "0.0.0.0:8100".into()
+}
+
+fn default_database_url_env() -> String {
+    "DIRECTORY_DATABASE_URL".into()
+}
+
+#[derive(Debug, Clone, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct TokenCfg {
+    /// Who holds it: what its snapshot is filed under and what the log says, never the token.
+    pub name: String,
+    /// No default: a token whose access is missing or misspelled is refused when the file is
+    /// read, rather than given some access nobody chose.
+    pub access: Access,
+    /// The token's SHA-256, in hex: what `ipx-directory token new` prints. The file holds no
+    /// token, so adding or revoking one is an edit here and nothing else.
+    pub token_sha256: String,
+}
+
+/// What a token may do, each level including the one before.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Deserialize, serde::Serialize)]
+#[serde(rename_all = "lowercase")]
+pub enum Access {
+    /// Browse.
+    Ro,
+    /// Browse and send snapshots.
+    Rw,
+    /// All that, and remove, block and refresh feeds and drop an instance's snapshot.
+    Rwa,
+}
+
+/// A token as the server checks it.
+#[derive(Debug, Clone)]
+pub struct Token {
+    pub name: String,
+    pub access: Access,
+    /// SHA-256 of the token, lower-case hex.
+    pub sha256: String,
+}
+
+impl Config {
+    pub fn load(path: &Path) -> Result<Self> {
+        let text = std::fs::read_to_string(path).with_context(|| format!("reading {}", path.display()))?;
+        Self::parse(&text).with_context(|| format!("in {}", path.display()))
+    }
+
+    pub fn parse(text: &str) -> Result<Self> {
+        let cfg: Config = toml::from_str(text)?;
+        let mut seen = std::collections::HashSet::new();
+        for t in &cfg.tokens {
+            if t.name.trim().is_empty() {
+                bail!("a token has an empty name");
+            }
+            // Snapshots are filed by name: two instances under one name would overwrite each other.
+            if !seen.insert(t.name.as_str()) {
+                bail!("two tokens are named {:?}", t.name);
+            }
+        }
+        Ok(cfg)
+    }
+
+    /// The tokens as the server checks them. A hash that is not 64 hex digits, or the same
+    /// hash under two names, is an error: which name a request is would be ambiguous.
+    pub fn tokens(&self) -> Result<Vec<Token>> {
+        let mut seen = std::collections::HashSet::new();
+        self.tokens
+            .iter()
+            .map(|t| {
+                let sha256 = t.token_sha256.trim().to_ascii_lowercase();
+                if sha256.len() != 64 || !sha256.bytes().all(|b| b.is_ascii_hexdigit()) {
+                    bail!("token {:?}: token_sha256 is not a SHA-256 in hex", t.name);
+                }
+                if !seen.insert(sha256.clone()) {
+                    bail!("token {:?} is the same token as another", t.name);
+                }
+                Ok(Token { name: t.name.clone(), access: t.access, sha256 })
+            })
+            .collect()
+    }
+}
+
+/// A token's SHA-256 in hex, as the config file holds it.
+pub fn hash(token: &str) -> String {
+    use sha2::Digest;
+    sha2::Sha256::digest(token.trim().as_bytes()).iter().map(|b| format!("{b:02x}")).collect()
+}
+
+/// Equal without saying, by how long it took, how much of the guess was right.
+pub fn same_secret(a: &str, b: &str) -> bool {
+    let (a, b) = (a.as_bytes(), b.as_bytes());
+    a.len() == b.len() && a.iter().zip(b).fold(0u8, |acc, (x, y)| acc | (x ^ y)) == 0
+}
+
+/// A new token: `ipxd_` and 256 random bits, the prefix so one found in a log says what it is.
+pub fn new_token() -> String {
+    use std::io::Read;
+    let mut bytes = [0u8; 32];
+    // Falling back to the clock would make a guessable token. Better to fail.
+    std::fs::File::open("/dev/urandom")
+        .and_then(|mut f| f.read_exact(&mut bytes))
+        .expect("no source of randomness for a token");
+    format!("ipxd_{}", bytes.iter().map(|b| format!("{b:02x}")).collect::<String>())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const GOOD: &str = r#"
+        [[tokens]]
+        name = "tower"
+        access = "rwa"
+        token_sha256 = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+
+        [[tokens]]
+        name = "friend"
+        access = "ro"
+        token_sha256 = "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"
+    "#;
+
+    #[test]
+    fn tokens_carry_their_access_and_take_their_value_from_the_environment() {
+        let cfg = Config::parse(GOOD).unwrap();
+        assert_eq!(cfg.bind, "0.0.0.0:8100");
+        let t = cfg.tokens().unwrap();
+        assert_eq!((t[0].name.as_str(), t[0].access, t[0].sha256.as_str()), ("tower", Access::Rwa, "a".repeat(64).as_str()));
+        assert_eq!(t[1].access, Access::Ro);
+        assert!(Access::Ro < Access::Rw && Access::Rw < Access::Rwa);
+    }
+
+    #[test]
+    fn a_missing_or_misspelled_access_a_reused_name_or_a_bad_hash_is_refused() {
+        let h = "a".repeat(64);
+        assert!(Config::parse(&format!("[[tokens]]\nname = \"a\"\ntoken_sha256 = \"{h}\"")).is_err());
+        assert!(Config::parse(&format!("[[tokens]]\nname = \"a\"\naccess = \"admin\"\ntoken_sha256 = \"{h}\"")).is_err());
+        assert!(Config::parse(&GOOD.replace("\"friend\"", "\"tower\"")).is_err());
+        assert!(Config::parse(&GOOD.replace(&"b".repeat(64), "xyz")).unwrap().tokens().is_err());
+        assert!(Config::parse(&GOOD.replace(&"b".repeat(64), &h)).unwrap().tokens().is_err());
+    }
+
+    #[test]
+    fn secrets_compare_whole() {
+        assert!(same_secret("abc", "abc"));
+        assert!(!same_secret("abc", "abd"));
+        assert!(!same_secret("abc", "abcd"));
+        let t = new_token();
+        assert!(t.starts_with("ipxd_") && t.len() == 69 && t != new_token());
+        assert_eq!(hash("abc"), "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad");
+    }
+}